Either can work if the funds are irrevocably committed at the time of the interview. Closing first is simplest; escrow released on visa issuance is the usual alternative when the buyer cannot risk owning a business he cannot enter to run.
Commit first, then choose the closing structure
The consular officer needs to see funds at risk on the interview date. If the buyer closes first, the business is owned, the money is gone and the risk is real; the downside is that a refusal leaves him owning a U.S. company he can visit but not manage from inside the country.
The escrow alternative places the full price with an agent under instructions to release to the seller on visa issuance and return to the buyer only on refusal; officers accept that structure because the only remaining condition is their own decision. What does not work is an escrow that also waits for financing, the founder's consent or further due diligence. Allow time for the consular questionnaire, document review and interview scheduling, which vary by post.
The founder's two-year management contract should start at closing regardless of the visa date, so the business keeps running while the buyer waits.