Help with TOGC

A client is selling their business as a going concern. They are vat registered. They sell houses and are paid commission for the sales. Part of the price of the sale of business as a going concern is for the commission on agreed sales ( say £3600 for the sake of this example). The house sale had not completed before the togc so therefore no sale (net commission £3000 + vat £600) has been put through the accounts, but has effectively been paid to them via the purchaser. Therefore the funds will be paid to the new business owner.
Would the seller be liable for the vat on the sale? (effectively putting the sale and vat through there accounts (cr sales and cr vat, dr trade debtors) as a debtor, cleared by the sale proceeds ( cr trade debtors dr bank) and the purchaser showing it as - purchase of business dr trade debtor cr bank, upon payment of commission for agreed sales cr trade debtor dr bank).
No vat charged on the sale of the business as sold as a going concern.