it doesn't come from sleeping on the grass AT ALL... there are many theories about where "sono al verde" comes from, but sleeping on the grass in absolutely not one of them. if you are interested you can read about them here: http://it.wikipedia.org/wiki/Essere_al_verde it might be a "mirror game" to look for information on the net, but if you know where to look and how to filter information, it might give you the correct answers instead of guesses out of nowhere.

Very very simple meaning: non ho soldi. It does not mean /I have not money with me now/, but /I don't have money at all/. More difficult is the origin of this idiomatic form; here is my guess: /Not money/ implies (in the long run) /not a place were to stay/ which implies / sleep outdoor/ that is / on the green grass/. Just a guess; maybe the internet will give you a better reason, but some times I prefer to look into my thoughts than searching through the Internet, which is fine, but sometimes just offer a kind of 'mirror game' where information (be it sound or a lie) is repeated over and over.

Sono al verde = I'm penniless! But, please, try to read this multiple explanation: http://it.wikipedia.org/wiki/Essere_al_verde