Jamieson, Fausset and Brown on Amos 7:3

3. repented for this--that is, of this. The change was not in the mind of God (Nu 2:19; Jas 1:17), but in the effect outwardly. God unchangeably does what is just; it is just that He should hear intercessory prayer (Jas 5:16-18), as it would have been just for Him to have let judgment take its course at once on the guilty nation, but for the prayer of one or two righteous men in it (compare Ge 18:23-33; 1Sa 15:11; Jr 42:10). The repentance of the sinner, and God's regard to His own attributes of mercy and covenanted love, also cause God outwardly to deal with him as if he repented (Jon 3:10), whereas the change in outward dealing is in strictest harmony with God's own unchangeableness.

It shall not be--Israel's utter overthrow now. Pul was influenced by God to accept money and withdraw from Israel.