During negotiation, before terms are settled. Escrow structure, security for deferred payments and post-closing control all have to be drafted rather than explained afterwards.
Raise it while terms are still being written
Buyers hesitate to introduce immigration requirements into a negotiation, fearing it signals fragility. In practice sellers deal with financing conditions routinely and a well-explained requirement is usually accommodated. What cannot be accommodated is a request to reopen signed terms months later.
The conversation should cover three things: how the deposit and escrow will be structured, what secures any deferred consideration, and who holds operational control after closing. Each answer needs to be in the agreement rather than in correspondence. Where the seller will not accommodate a necessary term, that is useful information about the transaction, and it is far better received before commitment.
It should also be understood that owning a U.S. business does not itself authorise the owner to work in it.