I'd be very careful about getting "more, supervisor duties" for the same amount of money. You could be taken advantage of like that. A kennel I worked for had me bump up to the assistant manager when she wasn't there-basically, I was in charge of everything 3 days a week (the girl only work 4 day) and I got no extra pay. If she screwed something up, and somehow it got left overnight and someone noticed it the next day-it was my fault! I had to deal with the same amount of "dirty work" plus deal with the customers complaints, horrible employees, and anything that happened with the animals. Plus inventory and stuff-please. I got rid of that promotion very quickly..they wouldn't give me more money.

Anyways, either you ask for the work release, or maybe you talk to her privately (never make it a big deal in front of other employees, always in private). And watch your back too, if she gets annoyed she could cause some trouble with your boss.