What is the ruling on fulfilling a vow to fast ten unspecified days, whether they be obligatory or supererogatory, with the intention of supererogation and the vow's wording being: "I vowed to fast ten days if I succeeded in such-and-such subject"?

Fulfilling a vow of obedience is obligatory, as Allah the Almighty says: {And let them fulfill their vows}, and the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him."

Therefore, it is obligatory to fast the ten days if success is achieved, and fasting them becomes a duty. However, if the condition is not met, then nothing is required of you, and fasting them is considered voluntary.

We note that a conditional vow is disliked, but it must be fulfilled when the condition it is tied to is met.