Rep. James R. Mann (D-S.C.) called "grossly exaggerated and biased" a story in Sunday editions of The Washington Post that he used his congressional office staff to aid a financially troubled coin company in which he was the principal financial backer.

Mam said yesterday his staff may have spent up to 15 hours pasting 15,000 postage stamps on $2 bills, fetching coins and performing other favors to keep the U.S. Coin Co. of Greenville, S.C., solvent.

"I find it difficult to account for as much as $100 in staff time, 15 hours at the most was spent working on this," Mann told United Press International. He said he would have responded the same way even if he was not owed about $38,000 by the company. Mann said, "You know, congressional constituency service involves all kinds of things. Hardly a day goes by we don't handle something purely personal for someone or relating to someone's business."

House rules forbid anyone from engaging in commercial ventures and other non-governmental activities in House buildings.

In an interview with The Greenville Piedmont, Mann said he would not request a retraction or correction because it would be "like trying to beat a devil around the bush." He said he was heartened by support from his fellow South Carolina congressmen and a telegram from Rabbi Baruch Korff, one of Richard Nixon's post-Watergate supporters.

A spokesman for the House Committee on Standards of Official Conduct, also known as the ehtics committee, said yesterday the committee generally investigated charges against a member of Congress only if asked to do so by House member. The committee had received no such request, said the spokesman.