Is it obligatory to pay the excess amount agreed upon later, knowing that it exceeds the amount originally agreed upon and the actual cost of the passport?

The fundamental principle is that falsification is forbidden, except in cases of dire necessity. If the passport is falsified and you are not compelled to it by necessity, then it is not permissible for you to obtain it, nor to contract for it, nor to use it. However, if it is not falsified, or if you are compelled to it by necessity, then paying the requested increase depends on the contract: If you rescinded the first contract and entered into a new contract with him for 1500 pounds, then you are obliged to pay the increase. But if you did not rescind the first contract and merely ostensibly accepted the increase to obtain the passport, then you are not obliged to pay it. This is based on the Almighty's saying: "O you who have believed, fulfill [all] contracts," and the Prophet's (peace be upon him) saying: "Muslims are bound by their conditions, in what is lawful."