I would not consider it a hassle if the allowance was already mailed. As discussed above, all you can do is file an RCE, so I will be doing more work on this case but I will be compensated for it. No big deal.
I would not consider it a hassle if I hadn't done anything yet. I might be annoyed that I thought I was done with the case with an allowance but now am not. This situation is unlikely, as most examiners are going to send an allowance ASAP once you agree.
I would consider it a hassle if I wrote up the allowance, it wasn't submitted yet, then you called to change your mind. I wasted my time writing the action, and I'm getting fewer counts now as I'm going to have to send a final instead. Like doop says, that is annoying. But again, not likely unless you reneg right away (which itself isn't likely in your scenario, since you are saying you are waiting for foreign approval anyway).
If the allowance was submitted but not mailed yet and you called to change your mind, it would be a hassle to track it down and stop the mailing process. I am not clear to what extent that is even possible, and that is extra work off the production clock to give myself fewer counts. I might just say I already submitted it and it's already out of my hands so I can't do anything, so file an RCE or CON when you get the allowance. I wouldn't feel bad about it since you were the one who reneged on the agreement. So ultimately, this scenario is a hassle if I'm super generous, but more likely not a hassle.
So overall, not that much of a hassle probably, could be annoying though...