Receipt?
How can the buyer prove that they paid for the firearm?
My situation was different but maybe it helps. My husband taught me how to fire a gun. There is a question whether you've received "weapons training", to which I answered "yes" and then I wrote something like "I bought my gun through a legal purchase on 03.2021. My husband taught me how to use it". When I got to the interview in Columbus, Ohio, and we went over the yes/ no questions I told the officer that I wasn't sure whether that constituted "weapons training" and explained it to the officer. He changed the answer to "no" and added "informal".
I guess what I am trying to tell you is you will have time to explain it at the interview. I also had the receipt from my purchase just in case -never offered to show it nor did the officer request to see it. Interview took 20' and I am now a citizen.
If I were you, I would try to get a copy of the receipt if you have it.