What is the expiation (kaffarah) required of someone who vowed to abstain from a sin for a specific period, then committed it, while having forgotten the details of his vow and what expiation he intended, and having delayed its performance due to not possessing the money?

Scholars differ on the vow to abandon a sin. The majority view is that such a vow is not valid because abandoning sin is already a religious obligation. However, others hold that it is valid and requires an expiation (kaffarah yameen) for anyone who violates it. Therefore, you must repent, feel remorse, and seek forgiveness. It is more cautious to offer an expiation (kaffarah yameen) for violating the vow. The expiation is to feed ten needy persons, or clothe them, or free a slave. If you cannot find any of these, then fast for three days. Forgetting the intention or not knowing about the expiation does not incur any consequence.