A person found a hundred dollar bill on Shabbos and stepped on it, intending to pick it up after Shabbos (Shabbos would be over in about half an hour). Another person who was sitting on a bench nearby walked by and said that he found it first. The one who was standing there felt bad and said, “OK, after Shabbos I will give you $50 since you found it first.” After Shabbos, he picks it up and sees that it’s fake money. When the person comes after Shabbos, is he obligated to give him $50?
It is clear that you didn’t mean to just give him $50, but only because it was half of the value of what was found. Since the value of what was fond is not worth anything, you don’t have to give him anything, as this is clearly what you meant. If you want you can now give him the “whole” $100.