No change in the till

On the 10/09/2018 I purchased an item which I returned today 11/09/2018 due to it not being required any longer so I requested to exchange the item with another item but it turned out the new item cost less than the original item which they agreed for me to exchange it but refused to give me my change, was told "we don't have change in the till" when I paid with my money in the first place