Deceiving and lying are not permitted in dealings with others

We have a shop selling auto parts. When we want to write the bill for the customer, we deduct part of the amount, but he asks us to write the original price on the bill, and he does that because the product is not for him. What is the ruling on that?

Praise be to Allah, and blessings and peace be upon the Messenger of Allah:

If this involves deceit and cheating, it is not permissible under any circumstances. It is haram for you to give him a bill stating more than the amount which he paid in order to take money from the state or any other body for this bill. This is deceit and cheating, and is not allowed.

And Allah knows best.