I think it might be most logical to delete the booking. The mare owner that has booked it is no longer using the breeding and the new owner will not be aware of the booking until they’ve bought the horse (or booking must be displayed on the horse page for all at some point, but I don’t think it’s necessary). The new owner will likely want to select a stallion themselves. Moreover, there’s a bigger chance of missing the booking because the new owner is unaware of it, meaning the stallion has an unused slot.
If the old owner of the mare wants to use the booking for another mare they can easily request/book the slot again after the sale I think.