Arrogant and so unproffesional

Brought two cars(Ford fico and Mecedes Benz) for full service and both cars were diagnosed,and quotation paid in full on the spot. The ford figo was fully serviced, Unfortunately for the Mecedes benz there was gear selector that has to be ordered from Germany on 26-09-2024 and waited for it 3 months for the part to arrived and on the 24th Dec 2024 the car was brought in for installation, the clinic called in after the installation, only to find out the problem was not sorted after collecting the car. Called the boss/owner and he said there is one little part that was not included in the package, Whose peoblem is that???????? Now has to order that little part and he is not going to pay for that. The excuse is the last time he ordered the part for another car it came with everything.What a lame excuse and unproffesionalism.