So the business does make a tax distribution? You’d originally made it sound like you were getting pass-through income with no distributions at all.
Having a retroactive sale (which IMHO is an iffy situation in general) is going to complicate that, because you’re only entitled to the distribution if you’re a shareholder on the record date. Assuming the record date was far enough past 1/1/2018 for the 2017 profits to be calculated and the amount of the tax distribution known, you may owe that early 2018 payment to the buyer if you have a 1/1/18 effective date. You need someone with some legal experience to read the corporate documents to know one way or the other.
And you’re taking the installment sale contingent on the future success of the business? And you’re doing all of this in an attempt to maximize financial aid, rather than because you want out? Picking a week when your accountant and attorney were available probably would have been a better choice.
The initial tax question was straightforward; the rest of it is impossible for anyone who isn’t privy to the whole picture to do more than speculate on, and speculation from random internet strangers is not what you want to be basing financial decisions on.