Is it permissible to fast days other than Mondays and Thursdays to complete a vow of fasting for a month, and is it possible to offer expiation (Fidya) instead of fasting due to social circumstances?

Embarking on a vow is disliked (makruh). If one vows obedience to Allah the Almighty, they are obliged to fulfill it in the manner in which the vow was made. It is not permissible to change or substitute this vow. There is no objection to the aforementioned variation in fasting if the wording of the vow does not necessitate immediate fulfillment. If one is temporarily unable to fulfill the vow, they may wait. However, if the inability is continuous and not expected to cease, then an expiation for an oath (kaffarat yamin) is sufficient.