Heres the information directly from the US embassy London website about what to do with your green slip.
If you have left the United States and are still in possession of the I-94 or I-94W it is in your best interest to forward it to the appropriate authorities so that your record is corrected and that you do not experience any problems on future travel to the United States as a result.
It is very important that you complete the back of the card listing the port of departure and date of departure from the United States and the carrier/flight information. The I-94 or I-94W together with a letter of explanation and evidence of your departure from the U.S. should be sent to:
DHS-CBP SBU, 1084 South Laurel Road
London, Kentucky 40744
USA
Do not mail the I-94/I-94W to the Embassy. Doing so will only delay the update of the departure information. The CBP office in London cannot update the arrival and departure system.
What evidence of my departure from the United States do you require?
The evidence can come from a variety of sources, including but not limited to:
Original boarding passes you used to depart the United States;
Entry or departure stamps in your passport to indicate you entered another country after you departed the United States (please copy all passport pages that are not completely blank and include the biographic page containing your photograph);
Dated pay slips or vouchers from your employer to indicate that you worked in another country after you departed the United States;
Dated bank records showing transactions in your home country to indicate that you were in another country after your left the United States;
School records showing your attendance at a school outside the United States to indicate you were in another country after you left the United States;
Dated credit card receipts with the credit card number deleted for purchases you made after you departed the United States to indicate you were in another country after you left the United States.
Please send legible copies or the original material where possible. If you send original materials please retain a copy for your records; the originals will not be returned to you. It will assist the CBP if you include an explanation letter.
http://www.usembassy.org.uk/dhs/cbp/i94.html