The World Federation One Stop Fiqh

Ruling 1542

If someone has been hired to keep the qaḍāʾ fasts of a dead person, or, if he has to keep a kaffārah fast of his own, there is no problem in him keeping a recommended fast of his own. However, if someone has to keep his own qaḍāʾ fasts of the month Ramadan, he cannot keep a recommended fast [until he has kept his own qaḍāʾ fasts]; and in the event that he forgets and keeps a recommended fast and he remembers this before ẓuhr, his recommended fast is annulled and he can change his intention to an intention of keeping a qaḍāʾ fast. However, if he becomes aware after ẓuhr, then based on obligatory precaution his fast is invalid, but if he remembers after maghrib his fast is valid.