It doesn't rule out negotiation. That's what the part about a written agreement is for.

It merely rules out pulling prices out of thin air. Which is what OpenAI is doing here, charging for an arbitrary amount of completely invisible tokens. The shady part is that you don't know how much of these hidden tokens you would use before you actually use them, thus making it possible to arbitrarily charge some customers different amounts whenever OpenAI feels like it.

See https://news.ycombinator.com/item?id=41535865

There's no problem if a specific sum is negotiated beforehand. Doesn't OpenAI bill at the end of the month post factum?