What is "exploitation"? What's wrong with profit? Have you been reading too much commie literature lately?
It's the capitalist concept of profit that's the problem. If I build something with hired help, I can sell it in exchange for other goods, like bitcoins. If I were a capitalist, I would then keep as much of the bitcoins for myself and give my employees as little as possible. Compare that scenario to one where I divide the bitcoins between myself and the other workers according to the amount of work we've each contributed because we act as partners. The latter scenario exemplifies cooperation while the former exemplifies exploitation.
I posit that these things are cooperative relationships.
Adam wants $500 per month more than he wants to use his basement. Bob wants to live in his basement more than he wants $500 per month. They cooperatively exchange these things to their mutual benefit.
If Adam wishes not to exploit Bob, he should offer his basement at no greater cost than the expenses Bob would incur, like heat, water, and general maintenance. Providing Bob with a dwelling is incentive enough for Adam to allow Bob to live in his basement. Taking more than that is exploitation.
Adam wants $6 in interest more than he wants to use his $100 this year. Bob wants that $100 now more than he wants $106 a year from now. They cooperatively exchange $100 now for $106 a year from now and they are both happier for it.
Adam's fair interest is that which Bob intends to accomplish with the $100. Demanding any more, beyond external transaction fees, is exploitation.