Must I fast three days as expiation for an oath, despite my mother's strong refusal due to her fear of hardship for me?

If you broke an oath, you are obliged to offer expiation for it, as Allah Almighty says: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever cannot find [each of these] - then a fast of three days. That is the expiation for oaths when you have sworn. And guard your oaths. Thus does Allah make clear to you His verses that you may be grateful." (Al-Ma'idah: 89). It is not permissible to obey one's mother in abandoning the expiation for an oath. However, one can act upon the opinion of some scholars that expiation is obligatory with leniency, which allows for delaying the fasting (if it is the only viable option) until vacation days, while noting that fasting is only sufficient when one is unable to feed or clothe.