If you're "pre-warning" him you're using the overrunning deliveries procedure.it's down to the DOM to sort out the cut off which i would have prewarned about
If you're doing that what reason will you give?
If the reason is that you're due back to work in the callers on overtime and he says don't worry about that we'll get it covered what's your response?
The original question was can I do this and the basic answer is yes if your manager is dumb enough to let you but if you're trying to say that you have a right to turn down one kind of overtime then at the same time demand another I'd love to see the agreement that backs that idea up...and there needs to be one because advising someone to do something just because you get away with it in your office isn't good advice at all.