Breastmilk is more stable than formula, meaning it will take longer to go bad. That said, from what I've heard, once a baby has taken some from the bottle there's bacteria that can grow from the introduction of his/her saliva. So the lactation consultants told me to throw away bottles he'd started after an hour.
I was in the same state as you though. I didn't have enough milk so my son got both breastmilk and formula and I never tossed a bottle that he started. I just put it right in the fridge and used it next time. My son was fine so it's really your call.
An idea if you're worried about it is to feed him a bottle with just breastmilk first to ensure he drinks all of that and then give him formula. If he doesn't finish that you won't feel as bad about tossing it (although that isn't cheap either).
(Oops and I keep saying him because my child is a boy, you have a girl...sorry, you get the idea.) :-)
Just sharing my experience, obviously it's your call.