While feasible, this situation is doubtful. It is a crime for both the transferee and transferor, so he'd be adding to his own list of charges.
It would also end up being a case of your word against his, and he's the one who has demonstrated dishonesty. Even if they're inclined to believe the criminal, all you need is one character witness who testifies in your favor, and mr. criminal gets the perjury bonus.
What you say sounds good but let's put it in these terms. A drug addict gets caught with a 3 ounces of crack. He tells the US attorney he will make a deal for a lighter sentence by giving up the guy he bought it from. Now he is admitting to the purchase not just the possession and he will even testify he made similar purchases from the dealer once a month for the past year. The dealer gets arrested, charged with the sale and is sentenced based on 36 ounces of crack (3x12). This is the reality of how the federal justice system works. You and I know how it SHOULD work, I'm telling you how it DOES work. In the case of the firearm the crook is already screwed because he has illegal possession of the firearm and is facing up to 25 years for it. He has nothing to lose by rolling over on you. His possession of the firearm without, according to him, a background check (the fact he has it supports that claim) is evidence you transferred it to him illegally. Along with his testimony you will have the weight of the evidence stacked against you and you will have to PROVE you didn't commit the crime. They have are suggesting you maintain the 4473 and maintain the paperwork as evidence of the transaction. If you can't get a hold of the dealers records, which the ATF has free access to not you (besides when you tell them where you did the transfer his records may be impounded by the same people seeking to convict you), and you can't produce your 4473 I can see the situation where you are truly screwed. I'm not saying this is the plan for all cases. I'm saying this can and will happen if this becomes the law of the land. I won't divulge my profession but I do review sentencing documents every day at work. You would be amazed now much weight is put on crimimals testimony when it serves the purpose of the state.