Leah,

My suggestion would be for two bottles to be prepared, one with
breastmilk, to be given first and the other with formula, to be given if
the baby still wants more after the breastmilk is gone.  I have done
essentially same thing, but with Lact-Aids, whenever I have had some
donated breastmilk for my adopted babies.  I try to make use of every
drop of "liquid gold", both that I can produce and that someone else
graciously provides for us!  I even rinse containers that have had
breastmilk in them with water to be used in mixing formula.

Aloha,
Darillyn