1)
What did Hashem tell the angel?
1.
'Rashi': He told him to return his sword to its case, and so he did. This is like "va'Yomer Hashem la'Dag va'Yakei Es Yonah" (Yonah 2:11).
2)
What is the significance of the plague ending in the place where the Beis ha'Mikdash will be built?
1.
Malbim (Shmuel II, 24:24): From now was fulfilled Nasan's prophecy. War from the enemy ceased, like I wrote above (Shmuel II, Perek 7); David began preparing the place for the Mikdash, and he did not need an army any more.
