What is the ruling on someone who vowed to fast on Mondays and Thursdays upon the fulfillment of a certain matter, and then forgot that matter?

If someone makes a vow of obedience conditional on a matter and then forgets that matter, if the matter materializes, he is obligated to fulfill his vow. However, if he does not recall what the vow was conditioned upon at all, or if he doubts its realization, then nothing is incumbent upon him; because the original state is freedom from liability, and a conditional vow must be fulfilled upon the certain realization of the condition or a strong presumption of it. Al-Izz ibn Abd al-Salam clarifies that the original state is freedom from liability unless the reasons for obligation are certainly established.