If the loan is denominated in Bitcoin and is interest free, on return of the loan amount, you have the same amount of Bitcoin.
If the loan is denominated in US dollars and is interest free, on return of the loan amount, you have the same amount of US dollars.
You would have have an unusual loan agreement for the outcome to be different.
For example if you loan your car, worth $70k to your friend and when they return it the car has a market value of $60k. Your agreement says your friend has to return 1.13 cars to you. And you also consider this a profit. This would be unusual. Presumably, if the car appreciated in value, you friend might only have to return 0.9 cars?
