Your options appear to be:
drop the issue
re-engage the negotiations.
If they have a lawyer and you don't you are NOT playing on a level field and your chances of success in the courtroom are far less then in a negotiated settlement.
You didn't specify the issues so there is no way to advise you about your best way ahead other than to recommend you see a lawyer if the issues are critical to you or the sum substantial.
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You options appear to be to accept the offer or sue. If they have an attorney and you don't, then you need to get one to help with this. The attorney will be able to better strengthen your position to the other party or explain to you how weak yours is. One thing to keep in mind is that why are they stopping negotiations? Do they not have any money? If so, you have to weigh the pros of getting paid at least something versus the cons of going to court and either winning or them not being able to pay anyway.
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