What is the solution for someone who vowed not to marry except a specific girl, and if he marries someone else, he would fast for three months, then he realized his inability to fulfill the vow or to feed?

One should avoid making vows (nadhr), because a vow does not avert anything; rather, it merely extracts from the miserly. The vow intended as an oath is that by which one seeks to encourage an action or abstinence from something. Its ruling, if one breaks it, is the choice between fulfilling the vowed act or offering an expiation for an oath. This is called "nadhr al-lajaj wal-ghadab" (vow of obstinacy and anger). The expiation for an oath is feeding ten poor persons, or clothing them, or freeing a slave; and whoever cannot find [these] should fast for three days. If the girl marries, then there is nothing upon you. But if you do not marry her, then you have the choice between fulfilling the vow (fasting for three months) or offering the expiation for an oath.