Another way to look at it is every exchange is necessarily unequal. Both parties gain else the exchange would not happen. So there is no expected "equality" in exchange.
Discussion
I've read human action and it correctly describes the mechanics of exchange. But it doesn't mean that every exchange is moral.
If there is a young woman who needs bread to eat, and I say I will give her money, but only if she has sex with me. She may agree. The money was worth more than her dignity. The sex (hypothetically, I would need do this) must have been more valuable to me than my money.
So what. That doesn't make it moral. That is EXACTLY the justification behind lending at interest. "Well, you agreed to it."
*Typo, I would never do this