Q: We have a mechanic workshop for repairing cars. Sometimes when we check a car and find many things that need to be fixed, for example, the gearbox may be damaged and it costs the customer a lot of money to have it repaired. We explain all this to the customer but the customer prefers to replace the old damaged part or gearbox with a new one because the price of the new one is closer to the costs of repairing the old one. After we fix the new part and the customer receives his car, we (Part No. 15; Page No. 143) give him the old part or the gearbox but he leaves it and refuses to take it as he does not need it anymore. Sometimes another client brings his car to be repaired. The car may need a new part or gearbox which may not be available in the market. We take the part from those that were left by previous customers and install them into the cars of the new customers. We ask the new customer to pay for it as a used part whose price is, of course, less than the new part. Are we committing a sin by selling the old parts which customers have willingly left to us? We appreciate your advice. We also have a department for repairing and manufacturing radiators. Sometimes we fix new radiators to the customers' cars if their radiators are damaged and unfit for use. The customers may leave the old radiators for us. Since these radiators are made of copper, we sell them as junk in return for three SR a kilo. Are we committing a sin by doing so? What is your advice in this regard? May Allah reward you best! We are waiting for your Eminence's reply.