I would not regard this relationship as a love affair. Sally could not say no to her master and owner. As I imagine it, Jefferson came into her room one night in Paris and took his droit de seigneur. 

Nor can I believe that a teenaged Sally would make some kind of bargain with Jefferson about the future of her children. How could she have anticipated that she would bear him seven children? He may have made such an offer to her, but she would have had to agree. 

Jefferson ruminated about when the "blood" of a mixed race descendant was "cleared" of the implicit stain of blackness.