Answer:
She picked a pebble and dropped it soon.
Discussion:
She drew out a pebble from the bag, and before looking at it she ‘accidentally’ dropped it into the midst of the other pebbles. She said to the loan-shark, “Oh, how clumsy of me. Never mind, if you look into the bag for the one that is left, you will 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.