LIKE THAT HOLIDAY plateful of fudge that the dog gulped down, the household supply of money probably vanished in no time flat this month. But while we're looking for a little more green to help get the red out, we note that cash is making a comeback of sorts in the marketplace. A good 2,000 stores and 15,000 people around town are participating in a "discounts for cash" program organized by a local savings and loan firm. Card-carrying members are getting 5 percent to 12 percent off regular prices when they pay with cash or checks instead of credit cards. And the way the math works out, there's something in it for everybody -- the buyer, the store and the savings institution.

There is no magic to it, for the cash customers are merely getting back some of what they would pay elsewhere to help cover the costs of the credit-card system. When cash and credit prices are the same -- as they are at so many establishments -- the additional costs to merchants of doing credit business are included in those prices. Credit customers get some of this back because they don't have to pay their bills for a month or more. But except for the relatively new cash-discount programs, most cash customers have gotten nothing back.

Under the local plan, the rebates don't go to the customers right away, but are sent by the stores to the S&L, where the money is posted to the shoppers' accounts and where it draws interest for them at the usual savings rate. The S&L benefits from these deposits as well as from fees it collects from participating merchants. Of course, any store in town can avoid the fee simply by starting its own discounts-for-cash plan; the difference is that the stores in the program receive advertising by being listed in a directory, which may generate new business.

None of this is likely to result in mass shredding of plastic, for the credit-card system is still popular with those who prefer to buy now and pay later, even if it costs a little more. Besides, given what inflation has been doing to prices, some consumers are content to pay later in what are "cheaper" dollars. Be that as it may, a successful rash of discounts-for-cash might slow down the accumulation of consumer debt just a healthy bit, while giving those who either cannot -- or prefer not to -- use credit cards a break.