At a later date, Person B returns a share to them. And B pays them for the privilege of borrowing the share in the first place.

Person B thinks that it will cost less to buy that share in the future than the money they make from selling the share today, which is why they asked A to lend them the share in the first place. They plan to buy a cheaper share and give that back to A.

A doesn’t care if they get back their original share, they just want to get a share back.