Refund

I bought 3 items from Adidas on the 11 October they were running a 3 for 2 cheapest one is free special. I ordered 3 items but the third one didn’t fit so I returned it, then I asked that they give me a voucher not a refund to buy the same item but a bigger size. They promise to refund both to this day I haven’t received anything from them when I called them they promised to refund but to this day nothing has been done. I’m fan of Adidas but they treated me badly when I called them telling me that I won’t get a refund because it was on special, I told them I don’t need a refund but a voucher to buy the same sneakers I returned whom were a smaller size. I’m really dissatisfied in their services I’m a very loyal customer but they made me think otherwise they were they are treating me.