In Sefer Melachim II, 18:14-17 it says that Chizkiyah offered to pay what Sancheriv will demand, and after he sent the money, Sancheriv sent Ravshake. In Divrei ha'Yamim II, Perek 32 it says that when Sancheriv came against Arei Yehudah, Chizkiyah stopped up the springs and river, and prepared many weapons and shields, and appointed war officers, and then Sancheriv sent Ravshake!
Malbim: When Sancheriv started to conquer the walled cities of Yehudah, Chizkiyah sent for Shalom and gave the tax that Sancheriv demanded. He intended for time to prepare for war. Right after paying the tax, he strengthened the wall and brought water into the city. This aroused Sancheriv to come a second time, and he sent Ravshake. Even though he made a Bris and took a tax, he considered that Chizkiyah annulled the Bris via preparing for war.
