My end buyer's financing just turned into a 90-day conventional loan and the seller thinks they have a cash close in three weeks.

The assumption doing the most work here is that the transactional funder will extend at all. Many term sheets for same-day or five-day holds include language that voids the commitment if closing slips past a defined date, and the funder's verbal flexibility on the phone rarely survives a 90-day ask in writing. Before the renegotiation math even matters, you need that funder's position confirmed in a written addendum, because if they walk, the A-to-B leg has no capital and the whole structure collapses regardless of what you negotiate with the seller.

On the renegotiation question: the predictable cost wins over the variable one, as a general principle. An extended transactional funding position compounds in a way a one-time price concession to the seller does not. If a 5k concession gets you a clean 90-day timeline with no funder on the meter, and the alternative is a funder charging 2.5 percent per month on 94k, the math favors the concession somewhere around month two even before you factor in the funder's right to exit. The problem is that going back to the seller with a timeline change and a price reduction in the same conversation creates counterparty risk of its own. The seller does not need to know why the close moved, but they will ask, and a weak answer that contradicts "cash in 21 days" can unwind more than the economics.

The risk you have not mentioned: title. A conventional loan on the B-to-C leg means the end buyer's lender will order their own title search, and the A-to-B transaction needs to season appropriately or the lender may flag the chain of title as a flip and apply anti-flipping guidelines, which some agency products still carry. If this is a full agency underwrite rather than a portfolio product, that is a material underwriting question, and the title company needs to confirm they have handled a double close with a financed B-to-C buyer before you rely on them to structure it correctly. That is a licensed professional conversation, specifically a real estate attorney in the state where the property sits.

What agency product is the end buyer's lender running, and have you seen their commitment letter?

The title company question is the one that actually resolves this before anything else, because a lot of settlement agents will not handle a double close where the B-to-C leg is a conventional loan with an agency underwrite, and if that is the situation here, the whole structure collapses regardless of what the seller or funder agrees to.

On the renegotiation math: if the seller concession to buy a 90-day close costs, say, 3,000 to 5,000 off the purchase price, that lands better than an extended transactional funding position that is both more expensive and contingent on the funder agreeing to hold at all. The predictable cost wins over the contingent one at almost any reasonable concession number here.

What does the end buyer's lender look like, a portfolio shop or a full Fannie/Freddie underwrite, because that determines whether seasoning or chain-of-title issues will surface at the B-to-C closing and blow the deal anyway?