billed more money than what I was supposed to pay

I ordered telephonically at work on 29 January 2016 my order was R228 I gave the delivery guy R400. I was in a hurry to get back to work I didn't even notice my change he gave me R90 change back. The money was short with R82 since then I was calling the store so they can give my money back they don't want the telling me they don't have prove that I was miss changed. I've been calling non stop I think I even used R200 airtime just for my R82 I really need my money back please assist