I'm not sure which answer you are asking for case law on but, parents can purchase their children guns and do the 4473 and give the child the gun. So that transaction was a wash either direction of the person filling out the form. ATF cares more about traceability than who coughs up the cash.
Case law where a purchase was made and paid for by one person to a gun owner who shipped a handgun from one FFL in state A to a FFL in state B and another person fills out the 4473 and picks up the handgun. I can find case law where person A buys a handgun for person B, who could not purchase said gun. One case that comes to mind is a LEO that bought a pistol w/ his discount and gave the gun to a relative (in another state). If I recall, the relative paid for the initial purchase and the pistol was handed to him across state lines w/o a FFL transfer.
I am not going to try to hide anything from the seller, who happens to be a FFL, I will ask him if he is okay w/ it. If not, I will just pay w my debit and be done w/ it and monitor my checking account closely for a few weeks. I have been hit twice in the last 6 months, once by an FFL in Texas after paying for an auction and my card was used in NY to pay for a taxi and food.