Jonah 3

"And Jonah began to enter the city on the first day’s walk. Then he cried out and said, “Yet forty days, and Nineveh shall be overthrown!...then God saw their works, that they turned from their evil way; and God relented from the disaster that He had said He would bring upon them, and He did not do it." vv.4,10

Read chapter 3.
Is this a contradiction of scripture?  Can God go back on His word?  It seems pretty clear of Jonah's words "40 days and Nineveh will be overthrown."  Spoken prophesies verses written ones can be understood differently by the hearer verses the reader.  There was no part of that message that told the Ninevites and their king to repent to be saved.  Yet this is how they responded.  They responded to it as a warning rather than as a "no way out"/ fate message.  We also see Jonah's response in understanding this as a prophesy of warning in 4:1.  We see other places in scripture where spoken prophesies get rerouted due to prayer and repentance.  This is not God contradicting Himself.  It's important to trust in God's words absolute if we can't do that than we have a untrustworthy God if He's always going to change His mind.  How do understand this apparent "contradiction" of God's words?  How can you continue to trust in His promises and not go back on them?  

(just a side note to this chapter: I love that the animals fast and wear sackcloth too (v.7-8);)