Inflation giveth, and inflation will taketh away -- or, at least, so the Reagan administration hopes when it comes to a new group of people who are about to become eligible for food stamps.

Under the law, the Department of Agriculture once a year must adjust the montly income levels that determine who is eligible for food stamps. To get those figures, Agriculture uses the "nonfarm income poverty guidelines" put together by the Office of Management and Budget.

Thanks to inflation, the maximum monthly income for a family of four to qualify for food stamps will, on July 1, rise to $705 from $621, according to a notice in the May 22 Register (page 27901). As a result, Agriculture believes that the new levels "may, at least temporarily, result in a somewhat increased number of partiipants" in the foodstamp program. That's an embarrassing situation for an administration that is trying to cut food-stamp cost in general.

The notice, however, points out a possible bright side to this situation that could be just as embarrassing. "These newly eligible participants wold be at the top of the eligibility scale," it says, "and since their incomes would rise due to inflation, it is not expected that they will be eligible for long."

No one is prepared to estimate how many new families will qulify when the higher income level goes into effect, or how quickly they will drop from the program. Thus there are no figures as to how much more money the program will cost.

Agriculture is trying to cut some corners, however. That May 22 issue of the Register (page 27960) contains a proposed rule change that, depending on which approach is selected, could save between $200 million and $400 million, according to department officials.

It's a simple notion that illustrates again that a food-stamp program that involves 23 million people and almost $1 billion a month in coupons has some excess money in it. Under current rules, whenever a new participant is qualified for food stamps, he receives his coupon allocation for the full month in which he applied. In other words, if an applicant filed on the next to the last day of May, and was subsequently found eligible, he would receive food stamps covering the entire month of May.

The department wants to change this, proposing that eligible applicants get an amount pro-rated to the date they applied, or that they get an amount equal to the quarter or half of the month in which they applied. Since the first two approaches are considered administratively complex, the half-month approach is the one Agriculture appears to favor.