While it sounds silly, COs accepting payment for doing favors for an inmate is not a good thing. That's the kind of thing that sorta needs to be a zero tolerance policy.
It probably has to do with the inability/expense of the facility to be able to properly secure outside food against other more serious kinds of contraband.
What compensation? He said that he only gave him enough money to pay for the food. If I give you $10 and say go buy me $10 of something there's no compensation.