1. Is an expiation (kaffarah) obligatory for an oath taken by someone who invited another to lunch and was not responded to, or swore to pay for the bill in a shop but that did not happen due to the other person’s insistence on paying? And is this considered a vain oath (yamin laghw)? 2. Is an expiation (kaffarah) obligatory for someone who took an oath and then forgot it and did not fulfill it for a period, then remembered it and fulfilled it?

A Muslim should fulfill his brother's oath. If he does not, he has broken his oath and must offer expiation, unless the oath was for generosity and not obligation, in which case there is no expiation. As for someone who forgot to carry out what he swore to do, and then carried it out when he remembered, there is no expiation upon him, because he did not break his oath. However, if he specified a time for his oath and missed it due to forgetfulness, then according to the Maliki school and the majority of scholars, he has broken his oath. But according to the Shafi'i school and one narration from the Hanbali school, he has not broken his oath, and this is the relied-upon opinion for fatwa.