How do you decide when to stick up for yourself and when to let things go?

Buddhism says you need to obey the country's law.

If the law says that it is an offence to rob, then it is an offence and you have to report it.

If the law says that it is an offence to damage others property, then it is an offence, and you have to report the matter, regardless of whether the property is your property or others.

Otherwise you are seen as a Buddhist conspiring with the offender, which is contradictory.

Of course, you can take the "see no evil", "hear no evil", "speak no evil" approach, but that is quite a high level practice, and takes many years to reach that level.

I think there are 2 conflicting Buddhist principles that apply here (or perhaps more, but I just don't see them)

FWIW I think that these are some other principles.

• This seems typical of the sort of material problem you may face as a lay householder. If you didn't want that, why aren't you a monk? if you aren't a monk, then perhaps you did want that?
• Be guided, not by your desires, but by your responsibilities: i.e. try to fulfill your responsibilities.
• I think (I'm not sure) that the Sigalovada Sutta implies you have (more) responsibilities towards those who are close[r] to you: your parents, your children, your teachers, your employees, etc.
• But another principle is to consider the welfare of others: if you "stick up for yourself" then would that harm others or would that benefit others? What about the attitude of "sticking up for others"?

When should you stick up for yourself and when should you let things be?

I think you might find theoretical or scriptural justification for both/either course of action.

Also apart from these two courses of action, is there a third, or a compromise? E.g. offer to share the cost of repairs? Or ask your lawyer's opinion?

As a householder and a lay person, I don't really feel that buddhist principles conflict with everyday life in any meaningful way.

Just because you see your material belonging as what they are - be it an aggregate of stones, wood, cloth, or other synthetic material does not mean you have to allow others to damage or steal them.

The pursuit of truth is ultimately what brings you dissipation and emancipation from conventonal attachments. You have no obligation to correct any perceived harm done to you. That is your self view arousing ill will. Kamma inforces itself. You don't need to act as an enforcer.

Whether to pursue damages or not pursue damages is entirely up to you. As long as you maintain wise attention by not acting out of averice, ill will, or restlessness.

Right Speech and Right Effort. Those are the two pieces of the Eightfold Path that will answer your question in regards to decisive action.

What is Right Speech?

What is Right Effort?

Right Speech is speaking for a purpose that is beneficial to all parties involved and at the right time and in the right way (nicely, usually). In short, Speak effectively for a universally beneficial cause at the most effective time.

Right Effort is taking efforts that increase the total good and reduce the total bad.

I hope the above Buddhist principles of action help to guide you to the correct behavior for the highest good of all.

The answer is up to you, since it will be you who affirms his power.

Letting go of the outcome means that you can be ok with either decision, I.e. You can act accordingly either way without becoming attached and overcome with grief. If you let it be, so you let it be. Do you think that would be wise? If you follow it up, that's ok too. In the next instant, the opportunity will arise again, and you simply follow what you believe and what is in your heart.

If it is in your best interest and strength to pursue the matter further, then it would be Buddhas recommendation to correct and act accordingly to your knowledge, and not be cowardly or lazy. After all, you clearly value your family and seek to protect them, so you should question the importance of the matter and for what purpose are you seeking to fulfill. If you or your family are not impacted whatsoever, could you still let it go?

If you continue following up, it may come as a surprise to what consequences arise. Even if you are patient with your own choices, you may still take action at a more appropriate time and remain happy. It is important to notice whether feelings of guilt, fear or grief arise with any choice, as they indicate attachment.

you said:

1. One shouldn't attach him/herself (to material matters).
2. One should move toward truthfulness in all things and correct harm.

but... the second one does not sound like a Buddhist principle... where did you get that from?

The first one (nonattachment, dispassion) is a universally valid Buddhist principle. As a rule of thumb I would stick with this one.

However, according to training I received, there are two places detachment can come from: 1) from strength or 2) from weakness.

1. when detachment comes from strength -- it comes from the idea of freedom, of independence, of being one's own island, of needlessness and emancipation, from wisdom, from compassion.
2. when detachment comes from weakness -- it comes from fear of confrontation or involvement, from laziness, from doubts, from incomplete understanding, from "spiritual materialism", from attachment to private peace of mind.

So you should look inside and see where your detachment comes from. If it comes from weakness then, while on the surface it looks like detachment, it really is a form of attachment.

If that is the case then you should definitely engage with that "someone" - not as much for the sake of what you see as "truth" from your side - as for the practice of overcoming your (hidden) attachment.