[X] Refuse the deal. Raid Brushcrest as planned. Whoever these People of Makar are, they are no danger to Greenvalley.
You guys have got to be kidding me!
You guys are given empty promises of future rewards by your eternal geopolitical threat, and you decide that's a good enough reason to spill your own blood fighting their other fearsome enemies, whom may I more are not and will not be a threat to us at least until we have expanded enough to deal with them. It's perfidious Albion offering future money to get it's enemies to fight each other in the neolithic.
Even if his offer was genuine, it would be geopolitical imbecility to go along with it. They are in a similar situation with us as we were with the white clans right after wolf/bear unification (which ended in our favour through wolf's diplomatic deception): one tribe has a much higher potential for population and the other has a stronger military. When we were small we were scared of the mountain clans but now we treat with them from a position of equality or even strength, given time undisturbed our inherently more bountiful land let us surpass them, likewise the lowlanders were a couple of refugees living in a burnt out husk and just a couple of generations ago they were constructing satellite villages to raid us from and keep us small contained and compliant. They are treating us like we treated our own pet clan of mountain folk, like a March to slowly and eventually assume control over. And look at where they are now, originally paid to fight our enemies and now their chieftain attends our council meetings and follows our religion. In a few generations they will be erased, having been subsumed into the clan. Will we be subsumed by a richer, more populous polity, be forgotten, be erased in the sea of grass ?
NO!
This beast was not slain in the cradle, we must now do so while it is yet in the crib for we will never have another opportunity before their tribe subsumes ours and crowds us out from access to the great lake by sheer population growth. There is a reason why farmers were the foundation of every high civilization. Once established enough the surplus food let's them drown out everyone else by sheer weight of numbers. Until we expand into the very same fertile farmlands they occupy along the river we cannot afford agricultural tribes to grow raid-free.
We strike now while they are distracted with raiders to the south.
Furthermore I propose that we make sure our hereditary enemy may never again rise up to strike at us by making it an everyturn habit for our warriors to burn their fields to make sure their population growth does not outpace ours.
Lastly if we are to embark on such a course of action we must make sure this envoy does not get back to them. We kill him and we march south.