Suggestions for unfinished BM bottle?

Sarah • I`m 35 and a new mom!
Occasionally my baby doesn't finish a breast milk bottle and I'm looking for suggestions to keep from wasting it. I had serious complications after deliver and recieved several blood transfusions which makes it hard to produce enough BM. I'm working hard to get my supply up and feeding her at least one bottle of BM a day and the rest formula. Needless to say, I hate to waste any BM if I can help it. Does anyone have suggestions on how I can keep from tossing the left over milk? I only give her the smallest amount that she normal eats but sometimes she will be satisfied before that is finished, and I won't force her to keep eating. What would you do?