Is it permissible to observe voluntary fasting instead of obligatory fasting in a case of waswasa (obsessive whispering) that a specific fast is obligatory, such as making up for missed Ramadan fasts or an expiatory fast for an oath whose details are not remembered?

A person afflicted with obsessive doubts (waswas) should disregard them and pay them no mind. The fast of someone who imagines they have broken their fast in Ramadan is valid, and there is no need for them to make it up; because the default is the validity of the fast until there is certainty of its invalidation. And whoever doubts whether they uttered an oath, or whether an expiation for an oath is due upon them, no expiation is due upon them; because the default is the exoneration of one's responsibility. And supererogatory acts of worship do not substitute for obligatory ones.