A year ago I bought an iPhone 7. 3 months later I exchanged it with my brother for a OnePlus 3T.

Now my brother wants it back. 😭

Time to teach him how the world works :D
You both agreed to a 1:1 exchange, which is basically the same as an contract of purchase between you two.
Since you own the 3T now, a repeated exchange would require a new contract. Since you can simply deny your consent he cant do anything about it.
However, if he's still underage, he could argue that he's legally not able to close such contracts and could get that exchange reverted in court :P
In plain: Kick his ass and tell him to screw off, that little bastard has to learn how the world works *evil laugh*

Sorry. Me being me..

And you refuse to exchange it. Trades over bro no.

@Kimmax haha I wish!

@Kimmax @skprog I can't say no because when we did exchange it, we had a clause which basically allows him to ask for it back. Which was a wise decision on his part. 🤷‍♂️

@puppypower you're a moron. That would never have come through with me as your lawyer.

@Kimmax apologies sir.