the phone is likely linked to an apple account. seems reasonable that if the components are approved for a specific phone and that phone is linked to an icloud account, that account could permit a swap?
That assumes that you managed to ask the previous owner to log into iCloud on another phone and "free" the previous one for repairs. I guess you could do that if he's buying a new iPhone from you, but still... it is similar to the Macs that are stuck on the previous owner's enterprise account.