What should a seller do if he discovers that a customer has claimed back 500 Riyals after first receiving the remaining amount, and should he pay the money from his own pocket?

The faithful hand only guarantees against excess or negligence. "Excess" means doing what is impermissible, and "negligence" means failing to do what is obligatory. If negligence in handling trusts leads to their destruction or loss, then a guarantee is obligatory. If your action is considered a shortcoming in your country's custom towards the buyer, then you are a guarantor and must pay the amount, and you may seek reimbursement from the man.