If the pacers are dry and not wet like they look then I would have to say it looks like a coating failure of some sort. But im with AC, it looks like a warer leak. My guess is a sprinkler line goes under the pavers... I found an iasue like this at a Coke facility in NY. Theirs was inside the office building but none the kess it was a water line leak. Jess

I would walk away from this. Like stated before there is another issue causing this.

Another instance where getting involved in someones concrete problems could be a disaster for you. Take the time you would spend on this and sell 5 houses or a condo complex instead. As I have said on other posts like this one, I speak from experience on this.