Bad customer service and faulty parts

Purchased a airmatic compressor for R5800 from them which was installed by RMI approved dealer who found that the part is faulty. When we return the item they immediately started to accuse us that the part was opened and tampered with which was not the case. They were not willing to listen and said that they wanted diagnostic print out to prove that it’s faulty. This is not professional there is no client service. They said its a new part but its faulty.