Commentary for Numbers 31:5

וימסרו SO THERE WERE HANDED OVER … [A THOUSAND OF EVERY TRIBE] — This serves to tell you in what high esteem the shepherds of Israel were really held — how dear they were to Israel. Up until they had heard of his approaching death what does it state? That Moses felt constrained to exclaim about them, “They are almost ready to stone me”! (Exodus 17:14). But when they heard that Moses’ death was associated with the execution of vengeance upon Midian (cf. v. 2), they refused to go to war until they were “handed over” against their own will (Sifrei Bamidbar 157:3).

וימסרו, to Moses by the representatives of each tribe.

וימסרו, even under protest, something that could be due to Moses’ death having been linked to the conclusion of this campaign (verse 2). Israelites, in order to prolong Moses’ life might refuse to join this expedition. (compare Sifri 157)