There is an argument that Banchero is better than Brown right now. He is a more efficient scorer (1.378 pps vs. 1.323 pps), better rebounder, more assists, less turnovers, with a much lower usage. Plus he is a lot younger and has a more manageable contract. Brown took a lot more shots, thus scored more.
There is 0 chance the Magic trade Banchero for just Brown. Boston would have to send other assets, especially since the salary doesn't match and the Magic don't have bad contracts to make up the difference.