Is there an idiom for someone who's doing you a favor with someone else’s property or money?
1 Answer
If the someone else was not willfully donating the money to the someone, maybe: rob Peter to pay Paul:
To harm one person in order to do good to another; by extension, to use money or resources set aside for one purpose for a different one.
Source: Dictionary.com
Your benefactor was robbing Peter (someone with money) to pay Paul (you).