Is it permissible for a Muslim to change a vow of obedience—charity—by increasing the amount before his wish is fulfilled, and is he then obligated to fulfill the latter vow or the former, whether it is permissible or not?

Once a vow is made, it is not permissible to retract it or substitute it, even if this is before the occurrence of the condition upon which it was contingent.

As long as the person did not intend separate vows, but rather intended to increase what he had vowed, he must pay the salary for three months. This is because his intention to substitute one month with two, and then with three, implies that he vowed an additional month each time, making the total of his vows three months.