It is obligatory on Muslims to be truthful in their transactions. It is not permissible for them to lie (Part No. 14; Page No. 276) nor to take people's money without the right to do so. Likewise, if someone is authorized to buy something, it is not permissible for him to take the extra price.
It is also not permissible for the person who sold the spare part to write an incorrect amount on the bill in order to betray the person who will pay for it. Doing so will cause him to pay more than the real value and this is a form of cooperation in sin and aggression and eating up people's money unjustly. It is not lawful to take the wealth of a Muslim except with his consent. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.