My profession is based on experience and practice. For example, if it happens that I come across a defect in a very expensive electric appliance, I can fix it by discovering the defect that might be very simple but which nonetheless requires knowledge of the theory of operation, as well as experience, to discover. Also, I might design an electric circuit for it that costs me very little money. How could I estimate the cost of maintenance?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.

Your question is one of a person who seeks the pleasure of Allaah and avoids transgressing the limits He has set. We supplicate to Allaah The Almighty to grant you His pleasure and acceptance.

In this case, you can calculate the cost of the materials which you put in place of the damaged parts, in addition to the fees that you deem suitably rewarding for your effort in the work and for thinking of a way to fix the defect. You should consider the following in estimating the return of your work:

Do not harm the other party by charging him what is too much for him to pay. The Prophet, sallallaahu ‘alayhi wa sallam, said: “There should be neither harm nor malice.” [Maalik] [Mursal (Disconnected)]

Therefore, do not harm either yourself or anyone else, so that Allaah would bless what you earn.

Allaah Knows best.