If he only uses 800MB out of the 1GB, then at recurrence time only 200MB will be moved, so you will have 1.3GB left next month.
If he uses 800MB wont he still have 200MB and therefore 800MB would be moved off "my" plan to take him back up to 1GB? So I'd have 700MB?
Ummm, yes. I’ve only had 2 coffees so far today...