Transfer without a background check is a serious violation that can lead to suspension or revocation of your FFL.
You must immediately have the buyer return with the item and acquire it into your A&D record from the buyer.
Then you need to properly complete a 4473 with the required background check. After you get a proceed response you will dispose the item back to the customer.
Although this will not “un-ring the proverbial bell” on the violation that occurred, it will go a long way to show the ATF that the violation was not willful and, pending a successful 4473, it will show that the item is not in possession of a prohibited person.