meal ticket

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A card or ticket entitling the holder to a meal or meals.
• noun Informal A person or thing depended on as a source of financial support.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A ticket or voucher that can be exchanged for food.
• noun idiomatic Someone or something that provides income or livelihood, especially as an exploited source.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a source of income or livelihood
• noun coupon redeemable at a restaurant and entitling the holder to a meal