The action in the original Assumption was probably as follows. Michael is commissioned to bury Moses. Satan opposes his burial, and that on two grounds: first, he claims to be the lord of matter, hence the body rightfully should be handed over to him. To this claim Michael rejoins: "The Lord rebuke thee, for it was God's Spirit that created the world and all mankind." Hence not Satan, but God was the lord of matter. Secondly, Satan brings the charge of murder against Moses. Having rebutted Satan's accusations, Michael then proceeds to charge Satan with having inspired the serpent to tempt Adam and Eve. Finally, all opposition having been overcome, the assumption takes place in the presence of Joshua and Caleb.
Michael and the Accuser Dispute Over the Body of Moses
Assumption of Moses 12 (lost ending, reconstructed)