Why does it say "va'Ma'alah", and not va'Hal'ah?
Rashi citing Bereishis Rabah 43:9: Avraham already enacted this - "v'Chelek ha'Anashim..." (Bereishis 14:24) - [Aner, Eshkol and Mamrei], who guarded the Kelim, receive a share like those who fought. Malbim - va'Ma'alah applies when there is a connection to what was before, and va'Hal'ah when there is no connection. Therefore, Chazal expounded that it was enacted earlier.
Why is this called Chok and Mishpat?
Malbim: Seemingly, it is a Chok (the reason is not known). David explained the reason, and made it Mishpat. Wars of Yisrael do not depend on their strength, rather, on their merits, that Hashem fights for them. 1 There is no difference between one who fights, and one who guards the Kelim and prays.
Malbim: This is why Avraham, who first enacted this (refer to 30:25:1:1), said "Harimosi Yadi El Hashem" - the victory is due to Him. Therefore, [the ones who guarded] "... Hem Yikchu Chelkam" (Bereishis 14:22-24)
