frustrated customer

I did may bye in Feb collected in mid May unfortunately lost my original receipt but I was told I can use my last receipt to exchange but when I went back 5days later to exchange I was told the about a different price on the system as I didn't have the original receipt I asked the manager to check using my partners ID number she said she cant...what puzzles me is a lay bye can be released using ID number but exchange of the same items its a different story I'm now stuck with wrong size items. I regret coming to your store, other store we exchange even without a receipt as long as the price tag is still there...im really frustrated as I wasn't asking for refund but same price exchange.