President Carter's new Secretary of the Treasury-designate said yesterday the United States "will not permit the dollar to deteriorate from its present level," and will beef up its $30 billion dollar-defense pool if necessary.

G. William Miller, still serving as chairman of the Federal Reserve Board pending Senate confirmation as secretary, said the administration "is committed if necessary" to bolster its resources to defend 03the dollar.

There have been reports that the U.S. might double the $30 billion fund.

Miller also reiterated his call to avoid rushing into a tax cut to counter the recession, recession, saying he does not "think today is the right time for any tax cut" on grounds that it might exacerbate the current inflation.

And he repeated, his earlier assurances that he would not be beholden to the White House staff in economic policy decisions - a key concern in the wake of Carter's recent cabinet shakeup.

Policy decisions are made by the president and myself," he said. "I don't believe the White House staff is going to get into major policy decisions. I have no fear there will be effort by staff to do any more than staff."

Miller made his statements at a hearing of the Senate Finance Committee called to consider his nomination as Carter's new Treasury secretary. Although the panel will not vote until next week, it appeared Miller will pass.

Committee members all but dismissed a Securities and Exchange Commission staff probe that reportedly concludes that the Textron Corporation, of which Miller once was chairman, made $13 million in questionable payments abroad during his tenure.

Sen. Russell B. Long (D-La.), chairman of the panel, said his own reading of the report had convinced him that "there's nothing in there that provides evidence of any wrongdoing" on Miller's part.

Other members of the panel also downplayed the report and instead praised Miller's qualifications for the cabinet job. Miller said yesterday he regarded the improper payments as "disturbing" but was "not involved in any way."

The actions came as, separately, the House Banking Committee issued a report urging Miller's successor at the Fed, Paul A. Volcker, not to raise interest rates here to help prop the dollar up.

Banking Committee Chairman Henry S. Reuss (D-Wis.) said boosting interest rates at home could worsen the recession and ultimately help drive the dollar down further. The Fed boosted interest rates last week to stem a dollar slide.

Miller's statement on the dollar marked the administration's strongest commitment so far to maintain the dollar's current value. On Thursday, Treasury Undersecretary Anthony M. Solomon hinted broadly the dollar-defense pool might be expanded.

Despite the firmness of Miller's pronouncements, however, the dollar turned in a mixed performance on the foreign exchange markets. Traders said there was some apprehension early in the day about the prospect of glum U.S. trade figures.

Miller also disclosed he has suggested to Carter a plan to exempt employers from having to pay Social Security taxes for the first six months on any new worker they hire who is under 21, but the plan is in preliminary stages.

The Fed chairman said he had proposed the new measure as a way to help ease the problem of teenage joblessness. However, he told the Finance Committee, "I don't know whether they're interested" at the White House.

Miller's call for caution on a tax cut was in line with his previous positions. He also urged that Carter continue to aim for a balance as possible" - in fiscal 1981.

Miller also advised the lawmakers that while he still believes a tax cut would be "premature" right now, they may want to begin making contingency plans in case the recession proves deeper than predicted.

And he reiterated earlier calls for special new tax incentives to help spur more business investment. CAPTION: Picture, Treasury nominee G. William Miller whispers to his wife Arianda before hearing. By James K. W. Atherton - The Washington Post