How can the cost of maintaining and repairing malfunctions in expensive machinery be estimated, when it relies on expertise and the personal design of an electronic circuit that costs only a meager amount?

You may calculate the cost of the replaced materials and add to it the compensation for your effort in repairing, while observing the following:

1. Avoid cheating and exploitation, as Allah Almighty says: (Do not consume one another's property unlawfully, but [instead] let there be trade [conducted] by mutual consent among you), and the Prophet, peace and blessings be upon him, said: "The property of a Muslim person is not lawful unless given out of his own good will."

2. Consider the interest of the other party, as mentioned by Al-Shatibi.

3. Do not harm the other party by demanding what is difficult for him to pay, for the Prophet, peace and blessings be upon him, said: "There should be neither harming nor reciprocating harm."