WORST EXPERIENCE WITH YOUR STORE PEP

i have purchased 2 items from Pep northgate, 3 days later i needed to change sizes on both items as it doesnt fit my son,as i couldnt find the purchase receipt i called PEP headoffice to ask if i can be able to exchange the items on sizes, the lady i spoke to confirmed that yes i can get the items exchanged without receipt s long as i do not want refund. i then went on to pep Bambanani mall to exchange, as i got there i was told its not allowed, explained that i called headoffice they said its allowed, but the lady at the till refused. so why wopuld i be taken from pillar to post with this, head office says this, the stores says that, which one is which exactly..