Is it permissible to observe voluntary fasts, such as on Mondays and Thursdays, while there are outstanding expiations (kaffarat) for breaking an oath that have not yet been fulfilled?

It is obligatory for a Muslim to guard his oath and not to make too many of them. If he breaks an oath, he must offer expiation for it: feeding ten poor persons, or clothing them, or freeing a slave. If he cannot afford this, he must fast for three days. It is not permissible to resort to fasting except when unable to perform the three aforementioned acts. It is permissible to observe a supererogatory fast before or during the expiatory fast, but it is better to hasten to observe the expiatory fast to clear one's conscience.