If you are paying a Mess Bill at your unit, and you incur another Mess Bill at a different unit (because you've been on a course there, for instance) you are entitled to a rebate of certain items on your parent unit's Mess Bill. Go and speak to your Regimental Accountant if you require further info.
Back in the early 90's, I was stiffed along with other PSI's and TA Snco's to instruct on Ex Fast Track - a TA pre commissioning course at Grantham.
Was there appx 4 weeks. Never lived in the Mess - don't think the live in members liked us much as the miserable b*stards tended to be completely overwhelmed by our keeness and smiling energy, we were lucky to get a grunt of acknowledgement out of the old dinosaurs - so we each had a room in the blocks near the lecture halls, but we ate all meals, had the tea and toast, papers, etc, in the Mess.
Was presented with the bill and paid up on the day we left as per usual, and presented receipt to RSM on return to Unit who deducted the same amount from our own units Mess bills. No dramas.
The same thing has happened after every other course that I have been on.