If I found a dollar bill, would it be illegal to keep it ?
No, it is not illegal to keep a dollar bill if you found it. However, if you found a large amount of money, such as a wallet with a significant sum of money, it is your legal obligation to try and locate the owner of the lost property and return it to them. Keeping lost property without making a reasonable effort to return it could potentially be considered theft or larceny, depending on the circumstances.

In general, it is always a good idea to try and return lost property to its rightful owner, as it is the honest and ethical thing to do.