Oh! So this is not about lending buying power. But about lending in the context of betting on currencies.

A lends 10 Xcoins to B. B puts 11 Ycoins into escrow. A will either get back 11 Xcoins or 11 Ycoins.

If Xcoins rise in price relative to Ycoins, B is happy and A is sad.

If Xcoins fall in price relative to Ycoins, B is sad and A is happy.