Bad service

I bought 2 items at the store amounting to R898.00 with a debit card. The sizes were unfortunately small. I went to refund the items on Friday the 28th then I was told I cant be assisted because they don't have enough cash, I need to ho to other stores or come back later. Today same thing happened, the cashier gave me bad attitude to tell me if they don't have enough cash they won't assist. My question is, why do stores treat black customers differently. I had to waste my own time anc petrol just to be given attitude.