Exxon Corp.'s new cash discount program caught Maryland's service station industry by surprise this week and set the stage for another showdown in the General Assembly between the oil industry and the state's retail gasoline dealers.

The program, which incorporates a credit card processing fee for dealers, points up a shortcoming in an otherwise successful campaign by the service station industry to ban the fees.

Leaders of the state's service station industry apparently failed to do their homework or ignored the fine print in the bill that supposedly bans the fees.

As chagrined leaders of the service station industry learned this week, the bill that became law last month places a limited ban on processing fees.

Under Exxon's new program, participating dealers nationwide will be required to pay the company a 3 percent processing fee on all customer gasoline credit card purchases. In Maryland, however, Exxon has implemented a program that apparently complies with the state's controversial law, which is being challenged in the courts.

Resourceful Exxon lawyers have interpreted the law to mean that oil companies may assess the fee if they convert customer accounts from open-end, or revolving, to closed-end accounts. Amoco will introduce a similar program next month, and other major oil companies may adopt the strategy as well.

"The law specifically permits it," said Buz Warfield, manager of Amoco's Washington District.

Although Mobil Oil Corp. said it isn't prepared to discuss any plans it might have, "It is something we're taking a very serious look at," a spokesman said.

Mobil reacted to the new law by curbing credit card operations in Maryland last month, and Texaco USA terminated its credit program in the state after challenging the constitutionality of the measure.

But Exxon, basing its decision on an interpretation from the consumer protection division of the Maryland attorney general's office, made the processing fee an integral part of its new discount-for-cash program.

Legislators "didn't address discount-for-cash, nor did they address closed-end accounts," insists an Exxon spokesman. "When the framers framed that law, they framed it within the usury section of the law."

He was referring to the new law that sets the interest-rate ceiling at 24 percent for most consumer credit transactions.

The intent of the law governing processing fees, Exxon maintains, is to prevent oil companies from passing on charges that could exceed the 24 percent usury limit on revolving credit plans. By converting all customer charge accounts to closed-end, or 30-day accounts, Exxon eliminates any risk of violating Maryland's usury law.

Credit card customers will be billed immediately after each 30-day cycle and will be assessed a penalty for late payment. If an account becomes delinquent, says an Exxon spokesman, "Steps would be taken to rescind your card."

The Greater Washington-Maryland Service Station Association contends Exxon "found a very convenient loophole" in the law. The trade group will attempt to close the so-called loophole but that could prove more difficult to accomplish than winning passage of the original bill.

William Shoemaker, executive vice president of GWMSSA, acknowledges that his organization had assumed until this week that the Maryland law banned processing fees in any form. "We readily admit that the law was improperly drafted," he concedes.

He says he hopes to set in motion a plan for correcting the situation before the General Assembly reconvenes. "We would hope that some legislator who was active in this before would prefile a bill," Shoemaker said.

But GWMSSA hasn't yet come up with a plausible explanation for its earlier miscalculation or misunderstanding of the provisions of the processing fee bill.

Dealers in Maryland may be hard-pressed to prove hardship resulting from their payment of processing fees under the cash discount program. They have the flexibility, after all, to pass the fees on to customers who use credit cards.

In a cash discount program, dealers may pass on the fee to credit card customers by establishing higher prices than cash customers pay. The cash price is usually 3 to 5 cents cheaper.

Up to now, dealers haven't produced any overwhelming evidence that consumers aren't willing to pay a fee for the convenience of being able to use credit cards.

By pressing for a law that restricts credit card operations, Maryland's service station industry may have maneuvered itself into a position that gives the oil industry greater marketing flexibility in the state.