You didn't buy scroll of return, you bought the spell of that scroll, which is like 200 scrolls in a batch applied on your armor. Just apply it on a free item (you can only apply 1 spell on each item) and the go to that item, you should see a green square on left corner of the item, click on it to use 1 scroll out of the 200 you bought. Or you could click b in your keyboard (Shortcut for combat backpack), then click on the item you applied the spell on to use the scroll.