A federal jury in Miami found that

Exxon

deprived 10,000 service station owners in 35 states of pricing discounts over a 12-year period and has ordered the oil company to pay $500 million in damages.

The judge may decide to include interest later, and that could bring the verdict to around $1 billion.

The lawsuit centered around a program by Exxon, which has since merged with

Mobil

to form

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Exxon Mobil

(XOM) - Get Report

, that charged cash customers less per gallon and increased the station owners' prices by 3 cents per gallon. Exxon promised to reduce wholesale prices to make up for the credit-card charge, but station operators claim the company only held up its end of the bargain for six months, manipulating prices to eliminate the savings after that point.

The shortchanging amounted to between 1.03 cents to 1.4 cents per gallon on more than 40 billion gallons, according to the owners. The owners claim they suspected they were being shortchanged, but could not tell from Exxon's invoices.

"We continue to believe that we provided the offset and gave dealers a very fair price and that ultimately on appeal Exxon's business practices will be vindicated," Exxon's attorney said, according to published reports.

Shares of Exxon Mobil fell 22 cents, or 0.3%, to $83.80 in Tuesday trading on the

New York Stock Exchange

.