Can Christians be guilty of "robbing" God like it says in Malachi 3?

Most certainly.

Luke 20: 45- 21: 4 tells the story of the poor widow's offering, contrasting the rich men who donate but a little pittance of their overall wealth wight he poor widow who donates her whole wages to God. Likewise, the rich and wealthy that donate say, 1% of their great wages, are effectively robbing God in the same manner.

This goes perfectly in hand with Matthew 19: 24 "Again I tell you, it is easier for a camel to go through the eye of a needle than for a rich person to enter the kingdom of God.” (ESV)

Since the rich value their money over their piety (or maybe even boast about the size of their donations when it is merely "pocket change" to them like the Pharisee and publican praying in Luke 18:10), they are effectively matching the description in Malachi 3.

The Gospels are quite clear on what the rich and wealthy (or really, any of us) should do: Matthew 19:21 “If you would be perfect, go, sell what you possess and give to the poor, and you will have treasure in heaven; and come, follow me.”

Luke 12: 33 "Sell your possessions, and give to the needy. Provide yourselves with moneybags that do not grow old, with a treasure in the heavens that does not fail, where no thief approaches and no moth destroys."

Luke 18: 22 "When Jesus heard this, he said to him, “One thing you still lack. Sell all that you have and distribute to the poor, and you will have treasure in heaven; and come, follow me.”" (all ESV)

Since, Matthew 25: 40 clearly states, ‘Truly, I say to you, as you did it to one of the least of these my brothers, you did it to me.’ (ESV) That is, every time one does not give to the poor and needed (or clothes the naked, etc.,) one is, effectively, robbing God in the manner of Malachi 3.

Hope this helped!

The New Testament is silent on the topic of Tithing. This left us with our own interpretation of the Old Testament laws regarding Tithe. Most of the Pentecostal and Evangelical denominations make it an obligation to Tithe while the Catholic church doesn't collect Tithes.

If we study the life of the first century Christians, they did not pay Tithes but instead, they sold everything they had and put it at the feet of the Apostles. They did not give 10%, they gave 100%.

All the believers were one in heart and mind. No one claimed that any of their possessions was their own, but they shared everything they had. With great power the apostles continued to testify to the resurrection of the Lord Jesus. And God’s grace was so powerfully at work in them all that there were no needy persons among them. For from time to time those who owned land or houses sold them, brought the money from the sales and put it at the apostles’ feet, and it was distributed to anyone who had need. (Act 4:32-35, NIV)

The NT simply say that you should give to God whatever you are willing and give it with a cheerful heart. But do not try to lie about the money to the church as Ananias and Sapphira did.

To answer the question, it actually depends on how you interpret the OT laws. Some take this verse from Malachi literally and pronounce blessings and curses. But the NT is silent on this. Remember that many laws from OT are now obsolete but it is not clear whether Tithing is obsolete or not. Nevertheless, the NT gives emphasis on giving to God and His servants.

Nevertheless, the one who receives instruction in the word should share all good things with their instructor. (Galatians 6:6, NIV)

So, give as much as you can. For the more you give, the more you will get. The less you give, the less you get. Don't just give 10%. Give 20%,30% or even 100% and God will bless you as He promised in Malachi. But I don't know about the curses.

Remember this: Whoever sows sparingly will also reap sparingly, and whoever sows generously will also reap generously. Each of you should give what you have decided in your heart to give, not reluctantly or under compulsion, for God loves a cheerful giver. And God is able to bless you abundantly, so that in all things at all times, having all that you need, you will abound in every good work. (2 Corinthians 9:6-8, NIV)

Tithing was a requirement to the Hebrew Nation by the Law of Moses. Jesus fulfilled the Law:

Matthew 5:17 Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil.

What we call the Old Testament at that time consisted of the law and the Prophets. So in effect Jesus is telling them and us that since Man was incapable of Keeping God's laws or even observing the requests of the Prophets, that he through his obedience would fulfill the requirements set forth in the law and the Prophets.

When Jesus fulfilled the law he also fulfilled the requirement of the tithe and offerings. In other words Jesus by his obedience, even unto death, had set aside all of the requirements of God necessary to inherit the Kingdom of God, and initiated a new covenant based on faith rather than obedience.

The amazing part of this new covenant, was that it took away any possibility of entering God's Kingdom through any of our works, and placed that obligation on having Faith that Jesus life, and his death on the cross, paid for all of the disobedience and breaking of the law and prophets and was proven by his resurrection to life.

Since Jesus fulfilled the law there is no longer a requirement to tithe, and let's understand that God never needed any material thing from us.

A group of us sometime back decided that even though there is no requirement to tithe, we would faithfully tithe ten percent of our earnings, and anything over that would be an offering. As tough as it seemed when we were only getting from one paycheck to the next, several of us began to tithe the ten percent as promised, and surprisingly we still got from paycheck to the next. Myself and a couple of others gave to some other things, such as toys for kids at Christmas time, some other needs that arose during the year. To our surprise we found that what we gave was not only mysteriously replaced but was multiplied. In my case the first year of giving in this manner was the first year in which I did not have to try to pay for our Christmas throughout the next year, and God has so blessed me that never again has Christmas left me having to scrape or pay for it the following year.

I tell you that story, because it leads to something very worth considering which is how much faith do we have that God will provide our needs, and even more than that do we place the pleasure of God above the riches of this World.

The answer to your question then is a resounding no! We cannot undo the fulfillment of the law by Jesus. We can only show our appreciation for the gift of Salvation by our participation in the work of God which includes financial help.

New Testament and Tithing While the New Testament is explicit and clear in establishing that much of the Old Testament law was no longer required after Christ's sacrifice was completed, it is, as has been mentioned, completely silent on the matter of tithing. So, while we can be sure that we need not follow the Law of Moses any more or circumcise or male babies... we are still bound to keep the law of tithing. The silence on the matter of tithing is more of a support for its continuance than a declaration of its abolishment. That the early Christians consecrated all their possessions to the apostles, as Mawia mentioned, is not evidence of tithing's abolishment, but rather that tithing is more of a minimum requirement in your Christian dedication. Finally, the practice of tithing, or giving a tenth of your income or possessions, predates Moses by hundreds of years, and therefore cannot be assumed to be included in Moses' law: please see Genesis 14:20, which explains that Abraham gave a tenth of his possessions to Melchizedek. Please see What does the Bible say about tithing

What about the cursing? While I agree with the other answers and am impressed by the faith and dedication of the Christians here who tested the Lord's promise through Malachi, I resoundingly say Yes, the downsides of not paying are also true. They are just the other side of the coin to the blessings. The Lord wouldn't uphold the blessings promised here but revoke the cursings also promised here: he is a God of order and consistency -- what He says goes. I think part of the cursing is that you don't receive the blessings promised. As Malachi implies, the blessings come in the form of protection from "the devourer" -- for a farmer, that's anything that would affect your crops. In our day, we receive this protection in subtle ways. For example, we may be blessed with health that reduces medical costs. Please see this address for an inspiring example of that.

So yes, from that perspective, not keeping the law of tithing is a curse: you may be keeping the Lord from "rebuking the devourer for your sake," whatever that devourer might be (i.e. not drought or plague but some other adversity).

Furthermore, the Lord expects us to obey his commandments. Grace is a gift, but it does not justify not doing what He has asked us to do. He wants us to test Him, to prove himself to us, which when it comes to pass, will increase our faith and bless our lives and cause us to glorify God. Not having that faith-building confidence is a curse in and of itself. As James said in James 2:18,

Show me your faith without deeds, and I will show you my faith by my deeds.

When we love the Lord, we do what His word requires us to do, with full purpose of heart. Who are we to do otherwise?