A divided Federal Election Commission has instructed an Illinois congressman to refund $7,750 in illegal corporate contributions he unwittingly received from a Chicago trade clearinghouse.

By a 4-to-2 vote, the agency said Rep. Marty Russo, a Democrat from Chicago, was obliged to pay back the money, part of nearly $30,000 that the Chicago Board of Trade Clearing Corp. funneled into the campaign coffers of five House Democrats in 1981 and 1982.

The firm was fined $100,000 Oct. 19 after pleading guilty to four misdemeanor charges of making illegal campaign gifts by padding employes' paychecks and having them make donations to the politicians, who were unaware of the scheme. Two former top executives were also fined and put on probation.

Russo had asked for the FEC's advice. Two weeks ago, with one commissioner absent, the panel tentatively voted 3 to 2 to order Russo to refund the money. As expected, Commissioner Danny L. McDonald joined two fellow Democrats and a Republican, Frank P. Reiche, to make the ruling final.

Dissenting were Republicans Lee Ann Elliott, the chairman, and Joan D. Aikens. Elliott said it was not equitable to penalize the congressman who came forward while doing nothing about the others who got the tainted donations.

The corporation, which handles accounting for commodity futures trading at the Chicago Board of Trade, also donated $5,000 apiece to Reps. Peter W. Rodino Jr. of New Jersey, Gillis W. Long of Louisiana and Frank Annunzio of Illinois, and $5,250 to former Rep. Leo C. Zefferetti of New York.