Compliant that store wouldn’t give a spare part of item customer admitted didn’t pay for

So call yesterday customer returned to one of our stores with a receipt. He had explained to store that the cashier must have forgotten to scan this item which is worth £200. He is missing a part from this item, store refused to give him the part as it wasn’t paid for so he called us demanding we send him the part. We advised him if he pays for the item store will gladly give him the part but we will not send him it out without proof of purchase. The audacity of some people! It was a colleague who got the call and I was supporting her.