I found a 100 dollar bill on the floor of the grocery store and go on to use it while paying for my groceries
Did you try to find who might have dropped it first, before you decided to pay for your own groceries with that money?
This is why I'm such a good person, I'm honest and just
The honest and just thing would have been to try to find the rightful owner of that money. If there was any indication that someone nearby might have dropped it, you should have asked them first.
So I shouldn't use it?
It's not a good idea to claim someone else's money as your own. However, in this situation, it might be impossible for you to find the rightful owner, so you can probably just use it if you need it.