“Oh, how clumsy of me. Never mind, if you look into the bag for the one that is left, you will be able to tell which pebble I picked.”
The pebble left in the bag is obviously black, and seeing as the loan-shark didn’t want to be exposed, he had to play along as if the pebble the daughter dropped was white, and clear her father’s debt.
Moral: It’s always possible to overcome a tough situation through out of the box thinking, and not give in to the only options you think you have to pick from.