Even on the surface this isn't a contradiction. The word used in Matthew carries the meaning of perish or neglect (Luke 11:42 "...and pass over judgment and the love of God..."); it does not automatically equate with destruction or passing out of existence.

Matthew, first of all, actually is an example of an oath and essentially means "even if" heaven and earth were to pass away (and they will not), so the idea is that Jesus' words will NEVER pass away. It is like saying, "When pigs fly."

Now look at Ecclesiastes in context. A wooden reading of this text could raise all manner of dispute with some interpreters. Such objections miss the proverbial nature of the comments, but also miss the point that this is a formulation of one of Ecclesiastes' "thesis statements" that is meant to provoke thought.

Our key verse, verse 4, "affirms the ephemeral character of humankind, against the background of the ever-standing earth." Or: "the permanence of the earth is merely the foil against which the restless coming and going of human beings is outlined." The theme is monotonous repetition, "an analogue to aimless and futile human existence."

Demanding a strictly literal harmony overlooks the genre of wisdom literature.