It depends on whether it was a loan when made (i.e. the expectation or plan at the time the money changed hands was to repay it, or that was the only condition on which the money was loaned), then you have to repay it, even without a written loan agreement--oral (sometimes called verbal) agreements are enforceable, too.If the money was a gift at the time it was made (no expectation of repayment), then you don't have to repay; a gift once given cannot be ungiven and does not have to be repaid.If you and your father-in-law disagree about whether it was a loan or a gift, he can sue you and let a court listen to both your stories, testimony, and any evidence, and then decide whether you have to repay or not.