@Shaye's:
Fossa is very beautiful - since she and Tillo get along, and you are already losing your heart to her, I would suggest you keep her - you are so wonderful with your pups, owned and loaned, that she would have a great home, and she really needs it. I have a "real" basenji and a mix, and am very happy to have both.
Ah yes, I remember the great times all of you had with Lycia. Tillo really did seem to enjoy his lady friend. But I agree with you that you should take a bit of time, at least until Wednesday, to think about this. Did you see yourself as a breeder down the line? Was the idea of getting a female down the line a step towards this? If so, you might want to wait, but until then, possibly do the best you can to find this dog a forever home, as mentioned above.
I don't envy the decision you have to make though. I don't know if I could turn down an immediate addition to the house, based on the situation you have mentioned. I know your partner has suggested you wait until getting another puppy, but with these feelings of adding another dog to your home, is it possible that you really are ready for another puppy girl right now? Just take the time to think it through clearly, and good luck.