What is the ruling on the fast of one who expels phlegm from his mouth, then collects with his tongue the remaining saliva on his outer lips and swallows it? And does this invalidate the fast, by analogy with Al-Nawawi's fatwa in "Al-Majmoo'"?

Expelling saliva or phlegm to the outer lip and then intentionally swallowing it invalidates the fast, and one must guard against it. However, swallowing it unintentionally does not invalidate the fast. Swallowing saliva without expelling it to the outer lip does not invalidate the fast by consensus. As for swallowing phlegm without expelling it, when it is possible to expel it, there is a difference of opinion, and the preponderant view is that it does not break the fast.