Hi - my name is XXXXX XXXXX I'm a Business litigation attorney. Thanks for your question.
The botXXXXX XXXXXne is that your husband has a right to enforce the contract as written. IF the partnership fails or refuses to pay the money as agreed, then your husband has a legal right to sue for specific performance and breach of contract and ask the court to enforce the contract and to compel the partnership to pay the money as agreed (to specifically perform).
However, it is fairly normal for a business to do things like this upon dissolution as it's a way to trim costs and hopefully leave more for the members after all matters have been settled. Also, it is very likely that the partnership has calculated what it may cost your husband to sue and fight for this money due him, and they probably think that he'd be willing to accept less to avoid legal fees and a protracted period of time to recover his money.
Thus, the partnership is basically fishing to see if your husband will bite - - and accept a lesser sum in exchange for avoiding a legal fight. Whether this is the case or not depends on what your huaband wants to do.
The best option would likely be to consult with a local business law attorney and have him/her give you a ballpark of what it will cost to pursue the claim, and then decide whether it's worth the effort or whether the offer is basically where you'd be in the end after paying your legal fees.
However, from a legal standpoint, if the contract is as clear as you say, he should be in a very advantageous position to prevail on the claims discussed above.
As for the interest, if the contract calls for interest to be paid out, then he would have a contractual right to interest. However, if there's no contract provision that says such, he could seek interest on the amount due him, but it would only be a realistic recovery if he were to sue.
It likely would be a good idea to at least consult a local attorney to see what it would cost to sue for the full amount. It may be that it can be done cheaper than you think and it may be worth it to do so.