The perception of warmth and competence in others influences social interaction and decision making. Virtual agents have been used in many domains including serious gaming and training. In this work we study the effect of warmth expressed in the behavior of a virtual agent on a human-agent negotiation. We design and conduct an experiment where participants negotiate with two versions of the same agent displaying varying levels of warmth. The results show that humans are more satisfied with the warm agent, are more willing to renegotiate with it, would recommend the agent more to their friends and had a better interaction experience, even though there is no difference in negotiation outcome (utility, agreement or rounds needed). While studies have shown effects of emotional displays on negotiation and collaboration, this is - to our knowledge - the first time that a clear effect of behavioral style is shown on the post-hoc appraisal of a human-agent collaboration, in our case a negotiation.