Must the seller pay the buyer's debt if the buyer claims to have paid but the seller has not confirmed receipt of the money?

It is not permissible for a worker or an agent to sell on credit unless authorized by their principal; otherwise, they are liable for the price. The sale by an absolute agent for عرض (goods/offer) or نساء (deferred payment) is not valid. If the buyer acknowledges the purchase and the deferment but claims payment, and the seller denies it, the seller's word is accepted with their oath, and the burden of proof is on the buyer. If the buyer fails to provide proof of payment, they are obligated to pay. If the bookstore owner authorizes the worker to sell on credit, the worker is not liable unless they are negligent or exceed their authority. If the owner does not authorize it, the worker is liable and can recourse the buyer for the amount.