Treasury Secretary G. William Miller told the Senate Foreign Relations Committee yesterday that a final agreement on a proposed multi-billion dollar International Monetary Fund substitution account is unlikely before next year.

Miller said the United States and other countries are making progress in negotiations on the substitution account, and these discussions will continue at a meeting of the IMF policymaking Interim Committee in Hamburg April 25 and at the IMF's annual meeting here tentatively set for Sept. 30 to Oct. 3.

Other U.S. officials said later that Miller wasn't really signaling a delay for the IMF substitution account because the process of formal approval for the plan by IMF member countries probably couldn't get under way until after the IMF annual meeting here.

The substitution account has been proposed as a way for the IMF to take in official holdings of dollars from governments and central banks abroad in exchange for a new IMF asset denominated in terms of the IMF's special drawing rights.

In his testimony, Miller devoted most of his time to another IMF issue -- the proposed 50 percent increase in member country quotas. The Carter administration is asking Congress to approve the plan to expand the regular financial resources of the IMF to 58 billion special drawings rights from the current 39 billion SDR total. Under an agreement reached earlier, the U.S. quota in the IMF would be increased to about $16.3 billion from about $10.9 billion.