1)
Why did he give a Mofes?
1.
Rashi: This was to show that his words are true.
2.
Malbim: (a) The Mizbe'ach was torn for a Siman that it will be Batel in the future. The ashes spilled to the ground for a Siman that the ashes of the Kohanim burned on it will not be gathered or buried. (b) If an action accompanies a Nevu'ah, especially if it is miraculous, the Nevu'ah must be fulfilled; it cannot be annulled.
