Is it permissible for me to own religious books given to me by customers at the grocery store where I work, knowing that the owner of the grocery store has given me permission to take gifts?

Exchanging gifts is permissible and encouraged to foster affection. The Prophet, peace and blessings be upon him, used to accept gifts and encouraged others to do so. It is narrated that he said: "Whoever receives a kindness from his brother, without asking or longing for it, let him accept it and not reject it, for it is a provision that Allah Almighty has driven to him." He also said: "Even if I were invited to a forearm or a hoof, I would accept, and even if a forearm or a hoof were gifted to me, I would accept."

Based on this, there is no harm in accepting books gifted to you, even without the knowledge of the bookstore owner, unless the giver owes you money. In that case, accepting the gift is forbidden for two reasons: 1. Deceit and fraud against the store owner, and the Prophet, peace and blessings be upon him, said: "Whoever cheats is not of me." 2. Every loan that draws a benefit is usury (riba).