In a show of displeasure over a White House tactic on one pending reform bill (on the U.S. Postal Service), the chairman of a House committee yesterday abruptly called off hearings on an unrelated administration reform bill (on civil service) scheduled for yesterday and today.

The committee rescheduled the civil service hearings for next Tuesday and Wednesday only after the White House did what the committee members wanted on the postal reform issue, according to a committee source.

Rep. Robert N. C. Nix (D-Pa.), chairman of the House Civil Service and Post Office Committee, told a crowd of spectators and witnesses assembled for yesterday's civil serice hearing that he was calling it off because of a "misunderstanding with the White House."

The problem had nothing to do with the civil service legislation. It was the result of confusion about President Carter's stand on the postal service legislatin, according to those involved.

Two members of Nix's committee also head subcommittees that focus on postal service legislation. They thought they had an agreement with the White House that it would not oppose the compromise postal service reform bill, which is expected to come up for a vote in the House soon, possibly today.

Yesterday's flap was prompted by a letter from Carter domestic policy assistant Stuart E. Eizenstat to key members of Congress. The letter out-lined Carter's "serious reservations and objections to the postal service bill," was well as commending certain compromises, which some membrs of Congress thought constituted outright opposition and contradicted the agreement.

A White House aide, on the other hand, said the Eizenstat letter was sent in the first place to scotch false rumors circulating around Capitol Hill that the president actually was supporting the bill.

This was all part of a long and complicated history of problems between the White House and Congress over certain elements of postal service reform, according to the White House spokesman, particularly on spending.

Yesterday's misunderstanding was resolved when the White House supplied Nix with a "clarification" of the president's position inwriting." While the president does not support the bill, it says, he will not oppose it either, according to a committee source.