Is it permissible to pay expiation for a vow to fast a month after fasting more than half of the period, and how much is this expiation?

Whoever vows to fast for a month must fulfill it. It is not permissible to resort to expiation unless there is permanent inability, such as old age or chronic illness. In such a case, one must offer an expiation for breaking an oath by feeding ten poor people, clothing them, or freeing a slave. If one cannot find any of these, then one must fast for three days, as stated in His Almighty's saying: "So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever finds not [the means] - then a fast of three days. That is the expiation for oaths when you have sworn. And guard your oaths. Thus does Allah make clear to you His verses that you may be grateful."