Very bad aftersales and customer etiquette

I purchased some tight for my wife. 2 days later i want to exchange the defective problem and accidentally misplaced the proof of purchase. When I reached the store they were willing to help me. But after I told them the tights were defective and they could not find the defect they basically said we will not exchange it because it is not defective and making me the customer out to be a ****. After that i purchase the same product and left with the defective product. After a while I found the defect and exchanged it also when they once again made me lair when they checked the system when i first bought it. I bought it on the 25 may and they said I bought it on the 23rd. Please just exchange the thing and stop making honest people to look like a ****. Very bad aftersales