The meeting only lasts for a few hours.
Most of that time was spent on sorting out the details of the agreement, seeking out loopholes, and ensuring all parties are satisfied with the final outcome of the contract.
Javier wasn't made to drink the Dark Guild Potion, but he was warned of the consequences of breaching the agreement. Because it is a written contract, denying any part of it would be difficult, and while such an illegal agreement is void by normal legal standards, it carries enormous weight within the Dark Guild.
Even Javier should recognize this; thus, the likelihood of his turning against this deal is low.
Once Snake concludes the agreement, with each party keeping a copy of the deal for themselves, he takes his leave from the building.
At first, he thought they might make an attempt to trail him.
But no such thing happened.
He was free to go.
