Neither extreme. Funds must be irrevocably committed before the interview, which is usually achieved by an escrow that releases to the seller when the visa issues, so the couple are bound but not stranded if it is refused.
Commit through escrow, close on issuance
Hypothetical example: a Frontenac Islands couple worry that if they close before the interview and the visa is refused they will own a Vermont campground they cannot run, and that if they close afterward the officer will see an uncommitted investment. The accepted middle path is a purchase agreement with the full price deposited in escrow, the couple obligated to complete, and release to the seller conditioned only on visa issuance. That satisfies the requirement that the funds be at risk and beyond their control while protecting them against an unrunnable business.
The calendar then runs backward from the season: the sale of the island farm-stay must close first to provide the funds, the escrow must be funded and the consular interview scheduled with time for issuance before the first bookings. The seller's cooperation on an escrow term, and on a short transition in which the seller trains the new owners, should be negotiated at the offer stage, not after signing. Hypothetical example: A Frontenac Islands couple negotiate to acquire a specialty pet-food manufacturer before its busiest season.
Their first review should decide whether the contract and escrow make the capital genuinely committed while avoiding an irreversible purchase if the visa is declined. The escrow agreement settles who controls release of the funds. Supplier transfers, permits, and seller training should be scheduled early because a complete purchase file can still be delayed by commercial conditions outside the immigration process.