What is the ruling on increasing the price of a spare part for a customer in exchange for purchasing, acquiring, and guaranteeing it, along with the technician's effort and transportation costs? And is it permissible to estimate the repair cost inclusive of labor and spare parts?

It is permissible to agree with the client on a known sum for the work as a whole, or to take the price of the item and a separate fee for its purchase, or a single fee for both. The guarantee of the item falls upon the inquirer if he purchases it for himself and sells it to the client. However, if he purchases it on behalf of the client, then the inquirer is an agent and is not liable except in cases of transgression or negligence.