My brother is doing some home renovation and has asked to borrow some money. I've loaned fiat to him in the past and he's always paid me back ahead of time and with interest. He's a trustworthy dude and I have no problems lending him money.

However, I don't have extra fiat laying around and I don't want to cash out any bitcoin. My thought is to loan him, let's say 0.5btc, and tell him that I want 0.5btc back in 6-18 months. I'm not looking for yield. I'm totally fine if bitcoin goes down in fiat terms when he pays me back - like I said, I'd just be hodl'ing anyway. It'd be dope for my bro if he got $15k worth but only had to pay back $12k.

My concern is if bitcoin goes up too much. If it goes up a bit and he pays me $20k on a $15k loan, that's no big deal - I'm sure he's good for it. But, what if bitcoin goes up like crazy and he needs to spend $100k to buy 0.5btc to give back to me? I'd feel bad to make him pay that back, but it'd also not be fair for me to have a smaller stack.

What do folks think? Will bitcoin not move too much over the next 6-18 months? Should I not loan bitcoin regardless of the situation? Any other risks to consider?