The Collaborative International Dictionary of English v.0.48:

Negotiatrix \Ne*go`ti*a"trix\, n. [L.]
   A woman who negotiates. --Miss Edgeworth.
   [1913 Webster]

WordNet (r) 3.0 (2006):

negotiatrix
    n 1: a woman negotiator [syn: negotiatress, negotiatrix]