We all have that relative that borrows money, then doesn't pay it back. They dodge and duck you for weeks or months, then randomly show up like a ghost in the night and pay you in full. They apologize and you actually start thinking they've turned it around. Then 3 nights later while clearly on a bender they either call and ask for a loan or bail money. You do it because they're family and you're a sucker. Then they don't pay on time, dodge you, ...................., etc
Point is, his family already knows he'll need it back shortly. Mom probably didn't even deposit the cash, just has it on the night stand waiting for drunk Marty to ask for it back