Is it permissible to use used spare parts that the questioner bought from a worker who sold them without the owner's knowledge and kept the money for himself?

It is forbidden for an employee to sell used parts in the shop without the employer's permission, as the benefit of a private employee is owned by the employer during the period of employment. If customers leave parts for the shop, they belong to the shop owner, and it is forbidden for the employee to sell them for himself. However, if customers leave them for whoever takes possession of them first, then the shop owner's instruction to the employee to take them to the shop and sell them makes them the property of the shop, and the employee is not allowed to sell them for his own account. The questioner should return the part to the shop owner and retrieve the price from the employee. If the acquisition of these parts is not part of the employee's work, he is not allowed to sell them for himself during official working hours.