You send the response with the Lametanian messenger, asking their King to provide legal proof for his demands.
810 AD, Autumn
4to6
The caravan of revenues from Augas arrives, and all the courtiers gasp at the size and splendor of it! The governor's assistant proudly presents the account ledgers and scrolls of taxation, all filled with rather large numbers. Coffers full of various coins are moved into the Treasury!
810 AD, Mid-Autumn
The reply from King of Lametania have arrived!
You ignore the first quarter, which is mostly insults, and concentrate on the latter, plus extra two scrolls describing some historical accounts, dating - phew! - back to 600's.
9!*
You go through the documents. They are all valid of course, but the claim by Lametania for the lands in question stands very weak today; it's been too long since Luathbas have acquired those lands, the territory back then was only partly Lametanian, and in your own royal archives there is also a record of a Lametanian chieftain passing his village to 'a Northerner '(definitely of Luathbas birth)'who saved his daughter from a giant red boar'.
Therefore, you can safely reject the claim on the legal grounds - you can expect the Lametanians to not accept this judgement, of course.
You write another reply for King of Lametania, in which...
A) You, despite the legal claim being rather weak, accept the request and agree to pass the southern territories, along with their population, to Lametania.
B) You explain that you read through all the documents, and reject his weak, if not obsolete, claim for the southern lands of Luathbas.