It basically means the money he gets while he is in prison he keeps. So if he works, he'll keep his salary. If someone puts money on his books, he gets to keep

It basically means the money he gets while he is in prison he keeps. So if he works, he'll keep his salary. If someone puts money on his books, he gets to keep that. If he didn't keep up with his payments to his debt he would be on a more restricted regime. While it seems like a small thing for us, it's a pretty big deal for his quality of life since he was never going to pay that back so for the rest of his life he would have had less prison money than he earned.