Sign before applying, with funds committed through escrow that releases on visa issuance, so the investment is irrevocable for immigration purposes without leaving the couple owning a business they cannot enter the country to run.
Escrow bridges the commitment and the decision
The application cannot succeed on a plan to buy; it needs a signed agreement and money genuinely committed. It would be reckless to close and pay the seller in full before knowing whether the visa will be issued. Escrow reconciles the two: the purchase agreement is executed, the funds are deposited with an escrow agent, and the release is conditioned on visa approval, with the deal unwinding if the application is refused.
Consular practice recognizes this arrangement as an irrevocable commitment. Sequence the rest around it: obtain manufacturer approval of the buyer as dealer first, since a refusal there ends the transaction; negotiate the seller's transition role in a way that leaves control with the buyer; then execute the agreement and escrow, assemble the file and apply at the consulate. Give the seller a realistic closing window that allows for consular scheduling and a possible request for further documents.
If the couple currently run their South Frontenac shop, do not list it for sale on a date that assumes approval; a conditional sale or a manager arrangement protects them if the timetable slips.