Is it permissible for the sponsor (kafeel) to take a sum equivalent to four months' salary in exchange for transferring the sponsorship (kafalat), and is it permissible for the sponsored person (makfool) to deduct this amount from the sponsor's money upon the transfer of sponsorship if he knew about it beforehand?

The first part of the question falls under the issue of interceding (bypassing due process using one's status/influence). The second part, which is taking money from the guarantor before injustice has occurred, is not permissible. However, if injustice has occurred, there is no harm in recovering an amount equivalent to this sum, and this falls under the issue of gaining victory/retrieving one's right by force if necessary.