I wonder if there's a way around this: instead of an explicit interest or profit, make it a trade-based market:

I'll give you X dollars-today, in exchange for 1.1X dollars-N-months-from-now.

Or I'll give you X dollars-in-a-month in exchange for Y Euros-today.

Does the SEC regulate currency trading markets?

And, if not, why can't dollars-today and dollars-6-months-from-now be traded as separate currencies [taking into account expected inflation/deflation, as well as the time-value of money]?