Question: If Zaid kept his qadhaa fasts for Ramadhaan, but only makes niyyat of the fasts he missed. He does not specify for which day he is keeping the fast. Is this correct to do?
If he specified that he is fasting today for a particular day, then without doubt it is permissible. If he without specification of the day of the missed fast keeps a fast of qadhaa, then too his fast will be valid.
Fataawa Mahmoodiyah Vol 15 pg.195