Rude salesperson and horrible service

We went to part to buy a tensionery the lady who assisted us checked the book and gave us a part,we went home gave it to the mechanic and he said it was not the correct one so we returned it the following day, she asked their mechanic and he confirmed that it was the right part.my mechanic left as he had to work somewhere else he recently returned and still said it was the incorrect part,this afternoon my husband went back there and they said yes it was the incorrect one, they took it back but then returned it saying it was without 1 ***** while as it was never fitted basically it was still brand new, I went to another shop where they confirmed that that part was brand new.so I fail to understand why they agreed that it was the correct part the 1st time around then changed the story and when they have to refund they come up with a lame excuse and bad and rude attitude.