Years ago when I was buying my first car my Grandfather found out that I was applying for a loan from the bank. He offered to lend me the money interest free, his point of view being that there was no need for the bank to make more money out of his family when he had plenty to spare. But he quite wisely insisted on a written agreement in place independantly witnessed in case our relationship soured before it was all paid back. Relations did not sour, but unfortunately he passed away only a few months into the agreement. The presence of this agreement helped with the division of his estate and undoubtedly avoided any family arguments over me getting out of repaying the rest of the loan as the outstanding amount was taken into account when calculating my share of his estate.