No return? Who's stock is it?

On Sunday 13/10/2019 we made a purchase at Mr Price Wonderpark. When we got home one of the swim shorts were the incorrect size(*** Meduim) I need a large. On Tuesday 16/10/2019 I went to Mr P Wonderboom Junction. I collected an online parcel and wanted to exchange the swim short for another size. Firstly you can't find anything because almost the whole store is moved at once. I asked one of the sales floor assistance where is the swim wear and she said they don't sell swimwear, I must go to Mr P Sport. During this I spoke to the assistant at the till and she said the Mr P is next door. Mr P Sport Wonderboom Junction said they can't take it back as this is not their stock, I must go back to Mr P. She phoned the other store and I just left. So what must I do now when shown from post to pillar? I want that swimshort exchanged and I do refuse to go back to Wonderpark when Wonderboom Junction is on my doorstep. And one last thing, if your staff don't know their products just look at it and ask for assistance from someone else.