What is the ruling on the prayer of someone who intended to pray the shaf‘ (even-numbered units) but then realized the Imam was praying the witr (odd-numbered unit), so he completed it with him? And is this considered a witr prayer for him or not?

If the questioner intended to pray the shaf' (even-numbered) prayer behind his Imam but did not intend the witr (odd-numbered) prayer, then what he performed is not sufficient for him due to the absence of the intention for the witr. It is recommended for him to make up the witr but it is not obligatory upon him. The congregant who only intended the shaf' has two options: The first is to form the intention for the addition (the witr) in his heart and continue following the Imam, even if the Imam intended to pray witr with three rak'ahs. The second is to sit when the Imam stands for the third rak'ah, intend to separate from the Imam, recite the tashahhud, and then say the taslim.