I think you are in a bad situation. Kinda sounds like they want the filly and next years baby but Momma is just the by product. Also sounds like they have no experience with raising a foal, much less foaling. I would be wondering what they are going to do next year when this mare foals. Clearly they have not thought it out that far yet. I am sure you are happy to have found a nice home for both of them but it seems these buyers dont know exactly what they are getting themselves into. I think you need to have a long talk and explain how it works. Yes, they might end up backing out of the mare, but better that then getting them home, realizing they CANT keep her weaned and being upset, upset with you, and /or end up getting rid of one or the other. Better yet you could help them along and SHOW them how to work with baby and Momma together and give them some advice and some book suggestions on how to be prepared for the birth of the next foal. This could be a disaster in the making OR you could be guiding some new people along and fostering a lifelong love of mini horses. Think of it as a wonderful opportunity to give back to those that helped you when you were starting out!!