What is the ruling on one who, while fasting in Ramadan, swallowed the saliva of a foreign woman out of ignorance of the ruling, then later learned that this invalidates the fast? Is he required to make up that day, and what about his repentance?

If you are doubtful about having swallowed a woman's saliva, then there is nothing upon you, because doubt after completing an act of worship does not affect its validity. However, if you are certain that this occurred, then you must make up the days on which you swallowed her saliva. Your ignorance that this is among the things that invalidate the fast does not affect the obligation to make up (the fasts), as long as you were aware of the prohibition of the act.