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There are no requirements to provide proof of onward travel when entering the Schengen area.
What you do need is adequate 'means of subsistence' both for the intended period of stay and for onward travel out of the Schengen area. Each Schengen country has their own guidelines when it comes to the required amount of money. Iceland operates with a guideline of 4,000 krona per day, meaning that you with 360,000 krona or about US$ 2,800 have enough money for the allowed 90 days. Even if you need additional money to buy a return ticket, you are well beyond the required amount if you dispose of US$ 25,000.