Brother received a $9k wire transfer, no clue where it's from

[pergesu]

On May 30th, my little brother got $9k wire transferred into his bank account. He's got absolutely no clue where it's from. He's called the bank twice, but apparently he has to talk to someone special, and they haven't been around when he calls. It's been a little while and he hasn't heard anything about it, so he's wondering if he can do anything with it. Let's say he spends every penny of it, and the bank realizes there was some mistake. Will he have to pay it all back, or is the bank held liable for the screwup?