Is the salesman considered sinful if he sold goods on credit and the amount was not paid by one of the customers, without the employer's knowledge, and what is the expiation due upon him after his return to his country?

It is permissible to sell a commodity for a deferred price that is higher than its immediate cash price, provided that both parties agree and specify the method of payment (cash or deferred) before the contract, to avoid "two sales in one sale." However, it is not permissible for an employee to sell on credit unless authorized by the employer. If the employee does so without authorization, he or she guarantees the value of the sold item. If your employer has authorized you, then there is no blame upon you, and you must inform him about the customer who has not paid. If he has not authorized you, then you are liable for the unpaid amount, unless the employer accepts the transfer to the customer.