Saahib-e-Tarteeb Forgetting To Make Qadha Of The Missed Salaah

If a Saahib-e-Tarteeb happens to forget about his missed Salaah and he offers the Salaah of the current time and remembers later on that he had missed his Salaah, he should perform his Qadha once he remembers, and the Salaah of the current time will be regarded as valid. For example, if one missed his Fajr Salaah, and he offered his Dhuhr Salaah and remembered only afterwards that he had missed his Dhuhr Salaah, his Dhuhr Salaah will be valid and now he should just make up his Fajr Salaah. 

However, if he remembers having missed Fajr Salah before or during his Dhuhr Salah, he must first pray his Qadha of Fajr, thereafter read his Dhuhr Salaah.