If I understand the situation right: seems like poor form to include an OOC offer in IC especially if it had no lead up. It's not as if the situation can't or couldn't be salvaged, depending on how deep you've gotten in the story or not. You could simply claim delusion on the part of one party who believed they offered a deal when they didn't which becomes a source of further interpersonal conflict. Or the non-offer of a deal that is later talked about as if offered becomes a character motivation for doing this battle in the first place; there may after all not be much justification for the one clan to fight another, so a reason is made up by the antagonizing character to invent a reason: "I did make a deal, trust me bro, I offered it to them, but then they disrespected me and my honor and ours. We should fight!"

That at least be how I would resolve the dispute, but it hinges imo on specific character details: would Merom Yuumei be deranged enough to dream up events that never happened and convince themselves they did take place and believe in them earnestly as real? Do they have the character basis and profile to be that way, have they been that way before, and will the writer keep that up? Or are they envious, scheming, and proud enough of themselves that at least in this moment they could see the benefits of making shit up to get what they want, have they done this before, and would they do it again?

If the answer to any one of these, or any combination is "yes", it can be saved. If "no" then you may have to work it out, maybe even go back to the point things went off the rails and do it all over to correct the record.