Can I add a condition that lets me walk away if a key customer doesn't confirm they're staying?

Yes — this is a common and legitimate way to protect a deal where a large part of the business's value is concentrated in one or a small number of customer relationships. The agreement can make closing conditional on obtaining a written confirmation, sometimes called an estoppel or consent letter, from the named key customer confirming its intention to continue the relationship on substantially similar terms after the sale.

The practical challenge is similar to conditioning closing on a key employee's agreement: the customer isn't a party to your purchase agreement and has no independent obligation to provide this confirmation, so getting it usually depends on relationship management and timing as much as legal drafting. It also raises confidentiality questions, since approaching a customer before closing can reveal that a sale is happening, which sellers are often reluctant to do too early for fear of spooking the relationship itself.

Because this condition touches sensitive commercial relationships, work out the timing and messaging carefully with the seller, and have the actual condition language — including what happens if the customer simply won't respond either way — drafted by a Treadstone business lawyer.