The Federal Election Commission has fined White House aide Margaret (Midge) Costanza $500 for her failure to file a required campaign fund report last July, according to documents released yesterday.

The fine represents the first penalty assessed by the commission to date for a late filing violation. It is also among the largest fines levied by the FEC in its three-year history.

FEC documents show that Costanza, through a lawyer, negotiated with the commission for over four months before [WORD ILLEGIBLE] to pay the $500.

At one point , Costanza's lawyer [WORD ILLEGIBLE] attorney on the case that [WORD ILLEGIBLE] client's White House job "seems to be a consideration" in the matter .

Costanza said yesterday, "I'm glad it's over."

She called the fine "excessive," and added, "I hope there is not a different level of justice for people who work in the White House."

Costanza had run for Congress from Rochester, N.Y., in 1974 and lost.

She ended the campaign with a deficit of $17,625, almost all of it owed to herself.

In April 1977, two months after she had taken her job as assistant to the president for public liaison, she held a $500-a-ticket cocktail party in New York City with Vice President Mondale as the guest of honor. The event took in $21,025.

Under the law, Costanza's committee was required to file a report with the FEC on the funds raised by and spent from the party by July 10.

No report was made, although Costanza's treasurer, Sandra Adams, who also is her White House assistant, had written the commission on late April that she had held a fund-raiser.

In August, Adams called the commission and arranged a meeting to work out the filing. According to FEC documents, she "failed to keep the meeting" and subsequent calls to her at the White House were not answered.

On Oct. 12, four days after a Washington Post story told of Costanza's New York party, the filing that had been due on July 10 was made.

By the time, an initial FEC staff investigation determined thatContanza's committee "may have committed a knowing and wilful violation" of the law.

In late November, however, after explanations from Adams and Contanza had been reviewed, the FEC general counsel, William C. Oldaker, recommended that the commission "should take no further action in his matter and close the file."

The majority of the commission on Dec. 1 disagreed. By a 6-to-1 vote, they decided to find "reasonable cause" that the reporting law had been violated and that Costanza should pay a fine.

FEC Chairman Thomas Harris, who cast the dissenting vote, did so, according to his aide, "because the commission had not taken similar action against other late filers."

Under FEC procedures a conciliation or negotiation process then began to determine the size of the fine.

The FEC staff recommended a $1,000 fine, but the commissioners cut it to $500.

In March, Contanza's lawyer wrote that his client wanted to "accept an agreement in order to demonstrate" that she "is being treated no more favorably than others."

But, he cautioned, the fine "could be misunderstood by the public. The inference arises that a more serious violation was involved."

He proposed $300, but the commission held fast at $500, and finally Costanza agreed on April 24.

Yesterday, Costanza said she would pay the fine with surplus campaign funds remaining from the April 1977 party. "It will reduce the amount I have distribute to charities," she added, noting that is how the remaining money - once her lawyer is paid - is to be spent.