In Mosiah 20, Limhi mentions an oath with the Lamanites.
Behold, my people have not broken the oath that I made unto you; therefore, why should ye break the oath which ye made unto my people?
When was that oath made?
And it came to pass that the king of the Lamanites made an oath unto them, that his people should not slay them.
And also Limhi, being the son of the king, having the kingdom conferred upon him by the people, made oath unto the king of the Lamanites that his people should pay tribute unto him, even one half of all they possessed.
It was made in Mosiah 19:25-26. The Lamanites promised not to kill Limhi’s people, who in turn promised to pay a 50% tax.