Does your money get credited back to your card?

I went to the kangaroo store and paid for $40 worth off gas off my debit card. I forgot I already had a little gas in my car so I could use the entire $40. However I had already paid for $40 and I had only pumped $34 when I feared it would be too much. I went back in the store to get my change but the cashier told me the money would be credited back to my card. Is that true? Will the money that I didn't spend go back to my debit card?