We have to be practical with some of these issues. Agree with him that you and him are done (if you are) and then work out an elaborate plan on how each of you will spend time with the young girl and support her (financially,emotionally etc.) without stepping on each others toes. Respect each other - It is possible to have respect even without affection - and everything ought to be alright.
And separate your work life from your 'domestics'.
Never count on making a good sale. Have the purchase price be so attractive that even a mediocre sale gives good returns.