Jet taking customers money by editing prices

I bought a shirt which was priced R129.99. (Later on saw on the slip that i actually paid R139.99) It was too small so i exchanged it. When i came to the tillpoint the employee called the manager to authorize. She saw the R10.00 difference but did not refund me the R10.00 that i paid extra, she just changed the new shirts price on the system to R149.99 and added the R10.00 discount to the slip. How is it okay to just take someones money? I dont care how small the amount is. How cam they treat people this way