The buyer's lender wants to see my contract on a single tenant retail box

Get the answer from the lender first, then decide. That order matters because a double close doesn't always solve it. Plenty of lenders order a full title chain and see the same-day A to B, and then you've spent 12k and still have the conversation.

On the seller reading. Commercial sellers, particularly an owner who's held a net lease box for years, generally understand there's a spread. He knows what he agreed to sell at and he had the chance to market it. The renegotiation risk isn't really about the size of your fee, it's about whether he has any leverage left to use. If your contract is clean, his contingencies have run, and his earnest is held, he can be annoyed and still close. If he has an out he hasn't used, then the number matters a lot.

So read your own contract for his remaining outs before you read his mood.

One more thing. Assignment disclosure requirements and whether the assignment itself gets recorded vary by state, and a few states have wholesaling-specific disclosure rules now, so confirm your state's with a local real estate attorney rather than assuming the residential norm applies. North Dakota extended its wholesaling requirements to all real estate rather than residential only, which is a sign the commercial side isn't sitting outside the trend.

If the lender is fine and the seller has no outs, disclose and close. The 12k is insurance against a risk you may not have.

@cobalt the point about reading his remaining outs rather than his mood is going in my notes. I'd assumed the awkward conversation was the risk and the contract was just paperwork.