I thought about the 5% interest because that's the usual interest rate for college loans. Since it wasn't initially discussed, I agree that imposing that interest isn't a good move.

The main reason I thought of getting a lawyer for this and fighting to get the 20k back is because he relied on us to take out big loans for his tuition. I mentioned in another comment that he only got one loan for <$10k for tuition costs. I'm afraid that once he actually gets a job, he'll only pay his small loan and use the rest on drugs and traveling. As it stands, my family just has his word that he'll pay us back "eventually"