Exchange policy

Can I kindly be assisted with the return policy of online store on goods that are on sale. I had a very bad experience at MR Price Kerk in Court Store where I tried to exchange a denim which I bought online with a sale price of R120 but worth R229 with a denim worth R199. I was told by a cashier that I had to top up with R79 which I refused because It didn't make sense that I had to top up on an exchange where I am leaving the denim which the store is going to sell for R229. Meaning the store is going to get R229 for that Denim plus R79 of my top up. She called the Manager by the name of Neo who told me that first of all online store is not the same with physical store. So I tried to explain why I dint understand the TOP UP I had to pay and my reason for refusing to pay it. He dismissed that saying I had to listen to what he had to say and and understand it so that I don't come back with the same issue. He told me that their store will make a loss if the exchanged for me since I bought the denim with a sale price of R120. I then asked him if they were going to sell the denim I am leaving for R120 but he said no. It was then that my confusion got worse because his answer was against what he said about the store making loss. So they refused to exchange for me without explaining how is the store making loss or showing me the policy that says I had to TOP UP.So what I want to know was the staff there trying to **** me or actually is the rule from Mr Price that we have to TOP UP for discounts when exchanging products that were on promotion I would understand if I wanted a refund of R229 whereas I bought it for R120. But it was a mere exchange