Lending money to a person gives you power over them
I'm not confident this counts as a trope. If this goes anywhere, it's Up for Grabs as usual and Needs a Better Description -- in particular, we don't want an Example as a Thesis.
Bob needs some money. You have money. You lend Bob some of your money. Now he owes you. Occasionally, the borrower is the one with the power. After all, he can't pay his debts if he's dead/in prison/whatever. The probability of this being the case increases with the amount of money owed. This is why you Never Lend to a Friend.