What is the ruling on praying upon waking to find a small amount of dry fluid on the underwear, assuming it is madhy due to its abundance, and is the prayer considered valid in this case?

If the fluid discharged is possibly either semen (maniy) or pre-seminal fluid (madhiy), it is considered semen (maniy) and necessitates ghusl (ritual bath). The Shafi'is permit a choice for the one who is in doubt, either to consider it semen or pre-seminal fluid, which is more accommodating for a person afflicted with obsessive doubts (waswas). However, if the probability is between three or more possibilities (semen, pre-seminal fluid, urine), ghusl is not obligatory.