Since becoming White House national security adviser, Richard V. Allen has received monthly payments from the sale of his interest in a Washington consulting firm, some of whose clients he has continued to meet with and advise on an ad hoc basis.

The payments, geared to retire a total debt to Allen of between $100,000 and $250,000, were revealed yesterday when Allen amended for the second time in eight days a financial disclosure statement he filed last February with White House counsel Fred Fielding.

It had been reported that Allen sold the firm, Potomac International Corp., to Peter D. Hannaford, a former aide and speechwriter for Ronald Reagan before his election as president.

It had not been reported that instead of paying a lump sum for Allen's interest or financing the sale through a bank, Hannaford entered a debtor relationship with Allen, agreeing to pay him on an installment plan, the terms of which White House officials would not disclose. This arrangement raises questions of whether a conflict of interest has arisen since Inauguration Day in Allen's regular contact with Hannaford, Hannaford's clients and a Japanese professor who had served Allen and now Hannaford as a conduit to some Japanese industrial leaders.

Allen and Hannaford were not available for comment yesterday.

Fielding's deputy, Richard Hauser, said White House officials had looked at whether Allen was in a conflict-of-interest position with Hannaford. He said they decided that "it's an argument that can be made, but I don't think it's all that real. If you own stock in Exxon, you hope they do well so the value doesn't go down, I suppose. But I looked at that side, too, and I don't think it is a prohibitive relationship."

In another development yesterday, a White House aide, who asked not to be named, said Allen "might well decide to step aside" if a special prosecutor is appointed to look into a $1,000 payment Allen received from a Japanese journalist. Also, Allen's personal secretary, Irene G. Derus, told a television reporter in front of her Chevy Chase home yesterday morning that Allen always intended to turn over to proper authorities the $1,000 thank-you fee given him in the White House Jan. 21 by Japanese magazine reporters. Instead, the money was put in a safe where it was accidentally discovered eight months later.

At least a dozen other National Security Council officials, including two secretaries working in Allen's West Wing office, said in interviews that Allen never mentioned the cash-filled envelope to them and that they learned about its discovery in the press.

Allen amended his financial disclosure statement for the first time Nov. 17 after a Washington Post inquiry to Fielding's office seeking to resolve conflicting statements by Allen and Hannaford on when the consulting firm changed hands.

Hannaford had said in an interview that the sale occurred last Jan. 18, two days before the Inauguration. Allen said on his disclosure statement that he sold the business in January, 1978.

In a note to Fielding after The Post inquiry, Allen said, "I provided the wrong date for one of the entries. Actually, I remained president of Potomac International Corp. until January, 1981, as opposed to the way it is stated: 'January, 1978.' "

Allen's correction prompted a review of the transaction by Hauser, who discovered that Allen had failed to state the value of his interest in Potomac when he sold it. During this review, Hauser said, he found several other minor errors and omissions, all of which were corrected yesterday.

During the review, Hauser and other White House officials discussed whether Allen's position as a creditor to Hannaford gave Allen a direct interest in the corporation's continued success. If that were the case, any assistance by Allen in providing advice or information useful to Hannaford's clients could pose ethical problems for Allen.

Several contacts between Allen and his former business associates in the last two weeks have been reported in detail. Allen also has acknowledged that he has met as many as four times this year in the White House with Tamotsu Takase, the Japanese professor and consultant with whom he worked jointly to advise Japanese automobile and industrial executives.

On March 23, Takase escorted Shoichiro Toyoda, the No. 2 official in the Toyota corporation, to Allen's White House office for a meeting that included discussion of delicate auto-import talks then in progress. As Potomac president, Allen had been a paid consultant to a Toyota parts manufacturing affiliate, Aisin Seiki Co. Ltd.

After meeting with Allen in March, Takase returned to Japan and, on April 15, briefed a symposium of Japanese corporate leaders on Allen's views of the import talks.

Allen also has met occasionally with Yasuhiko Suzuki, Washington vice president of Nissan U.S.A., the marketing company for Datsun. Suzuki had paid Allen a reported $100,000 annual retainer from 1977 to October, 1980.

Because of Allen's relationship with Datsun, White House officials have said that he did not participate in decisions earlier this year aimed at reducing levels of Japanese auto imports.

However, the White House later acknowledged that Allen sat in on meetings between President Reagan and then-Japanese foreign minister Masayoshi Ito and former prime minister Takeo Fukuda, both of whom discussed the import issue with the president.

Allen has maintained an especially close relationship with Hannaford that, according to Hannaford, has included regular contacts, dinners and some consultations about the proper "protocols" for Potomac clients. One source said that Allen and Hannaford had met with at least one foreign diplomat since Jan. 20 to introduce Hannaford as a "contact" man to the administration.

Sources in the international consulting community have said in interviews over the last several months that Takase has represented himself in Japan as someone with access to the Reagan administration by virture of his close relationship with Allen.

Meanwhile, a White House aide stressed yesterday that no decision has been made on whether Allen would stay on if Attorney General William French Smith decides that the case should be turned over to a special prosecutor under the Ethics in Government Act.

"I think it is fair to say there are many in the White House . . . who feel that Dick himself might well step aside . . . but I'm not aware of any decision being made," the aide said.