Bribing the enemy commander would be an acceptable roleplaying explanation as to why Hayasdan has just handed over a newly-conquered province to Pontus for nothing. Pontos has gained a new province without conquest. Provided that FriendlyFire actually sends a diplomat to the Hayasdan army commander and offers a very substantial 'gift' of several thousand mnai.
Another acceptable reason would be Pontus threatening war against Hayasdan if it didn't hand the town over (equivalent to 'Accept or we will attack' diplomacy.) Provided that Friendlyfire sends a diplomat to the Hayasdan commander, and cancels the alliance between Hayasdan and Pontus - since after a threat like that, realistically, Hayasdan would no longer want to be allies with Pontos.
(Actually, to do the above, I'd use Force Diplomacy to first give the town to Hayasdan, then use FD again to force Hayasdan to give it back under a 'Accept or we will attack' offer. This will create a proper diplomatic 'transgression' in the game engine, making Hayasdan very angry toward the player for the rest of the campaign, and will also give the other AI factions warning of Pontic treachery toward its allies, making it harder for Pontus to gain allies in the future.)
This would be an historically accurate reflection of ancient diplomacy - gaining a new province through treachery (especially towards an ally) is actually much worse, diplomatically speaking, than conquering it in open battle. Battle is honourable - treachery is vulgar.
It's a bit like the way Hitler got the rump of Czechoslovakia in March 1939 - he got a province for 'free', without a fight, but he also paid a price - he lost his reputation as an international statesman, so that no-one ever trusted him again (particularly Britain went from 'unsympathetic neutral' to 'hostile potential enemy'.) All actions have consequences!
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