Someone could have been skimmin' money from their pimp...
That lady would not be happy that you took the money, but on the other hand, if you somehow returned it, I'm sure her pimp would be just as upset if he found out she was skimmin', so for her sake keep the money, so her pimp never finds out.
The pimp hand falls hard in Detroit, I know I wouldn't want to cause trouble here, you probably don't want to cause any more trouble in your neck of the woods either. Keep the money.