When someone says "we will not renegotiate", they mean they won't make a deal "worse" for them than the one already made.
If someone comes along and gets a worse deal than that already agreed, you can't call it a renegotiation!
Seller: Ok, ok, £500 less. You can have it for £3,499. NO RENEGOTIATIONS.
Buyer: Errrm, I want to renegotiate that I'll give you £3,800 for it.
Seller: Errrm, ok!