Let the girls figure this out on their own.
Your little one decided that $5 was worth it to have her big sister's time and this was worth it to your big one. Shrug. I don't think this is something to be upset or angry or sad about.
I really wish people would let their kids 'handle' these things on their own. When we put our own emotions away and let the kids sort it out, they learn *so much better* from these experiences than we could ever hope to 'teach' them with our interference. I wouldn't force your bigger one to do something she doesn't want to do.
For what it's worth, my son used to ask me to drop everything and go to the store because he had money to spend. I'd tell him that sure, I would, if he would compensate me for the price of the bus tickets and do the task that I couldn't do while we were out doing his thing. He declined. Everyone has a price when it comes to doing things they genuinely don't want to do, because then it is a job/work... if it were me, I'd just tell the girls "well, glad you both worked something out" and leave it at that. Stay out of it.
Oh, and the idea of having big sister give the money back? Nothing teaches a lesson like buyers remorse. Your eldest likely figured that this was the equivalent to babysitting. Let them learn on their own. Sure, talk about being wise with money... but we don't bail our son out when he makes a poor purchasing choice. Wouldn't teach him anything except that mom and dad fix everything. Ugh.