Is it permissible to secretly take more money than agreed upon from a person who threatens to extort a larger sum, in order to avoid his harm, especially given the difficult circumstances?

It is not permissible to sell visas because they are a contract of donation, not of exchange. However, it is permissible for the person who obtained the visa to take an amount equivalent to their expenses and wages for their work. It is obligatory to give them what was agreed upon, based on the Prophet's (PBUH) saying: "Muslims are bound by their conditions in what is permissible." If they ask for more, it is not obligatory to give it to them. It is permissible to take the excess from them secretly (the issue of al-Zafar) provided that the right is established as a debt owed by them. Threatening them does not permit theft.