But it was legally transferred to his brother in another state. Legal possession, not ownership, lies with the brother. To get it back across state lines an FFL has to be involved for the transfer back to the owner. Otherwise, the brother has violated the following section of code:
18 USC 922
(a) It shall be unlawful—
...
(5) for any person (other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector) to transfer, sell, trade, give, transport, or deliver any firearm to any person (other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector) who the transferor knows or has reasonable cause to believe does not reside in (or if the person is a corporation or other business entity, does not maintain a place of business in) the State in which the transferor resides;
By the time all the shipping and transfer fees accrue, not to mention the agravation, it may well be cheaper to lend the brother the money to buy a similar gun. When the brother is able to afford the gun he really wants, just sell the temporary gun and give the money back to the lender. The lender would then be ahead of the game.