Due to his marrage to Tuon, Mat is now the heir to the Seanchan throne. The Seanchan would presumably have representatives at the FoM, but either due to the second attack on Tar Valon or her sence of importance it is likely that Tuon herself would not be present at the FoM. Therefore, Mat would be the highest-ranking Seanchan there, and thus able to agree to Rand's terms on behaf of the Seanchan. This would also appariently be in keeping with how the Prince of the Ravens acts, as in one of the books Tuon thinks that having a Prince of the Ravens that she does not have to plot against is too good to be true.
Another option is that Mat's title refers to power he has over the Deathwatch guard and the Seekers, who are tattooed with Ravens. If Mat has some influence over those forces, he could use that to force Tuon- who has less power than a usual Seanchan empress- to agree to Rand's conditions and to refrain from takin Wise Ones or Aes Sedi captive.