Private U.S. grain companies have resumed wheat sales to Iran that were terminated after the American hostages were taken, according to an Agriculture Department official.

Richard J. Finkbeiner of the department's foreign agricultural service said the grain is believed to be the first sent directly from the United States since the U.S. Embassy in Tehran was overrun in November 1979.

Finkbeiner said the Treasury Department recently published official regulations clearing the way for normalization of U.S.-Iran trade. The Carter administration embargo on U.S. sales to Iran did not apply to food and medicine, but the International Longshoremen's Association refused to load ships for Iran while the hostages were held.

On Capitol Hill, Deputy Secretary of the Treasury R.T. McNamar told the Senate Foreign Relations Committee yesterday that Iran appears to be acting "responsibly" in negotiations regarding fulfillment of the terms of the financial agreements between the two nations that ended the 444-day captivity of 52 of the hostages.

McNamar said that he is convinced that Iran has a "tremendous economic motive" for honoring its end of the financial agreements.

Undersecretary of State Walter J. Stoessel, in testimony before the same committee yesterday, said the United States has made representations to Iranian authorities about the status of the embassy compound, where the Americans were seized on Nov. 4, 1979. The student militants who seized it recently announced that it is being given to the poor.

Stoessel said the United States had asked through diplomatic channels that the compound, which is American government property, be placed in the care of the Swiss government, which represents U.S. interests in Iran. The State Department official said "the possibility is definitely there" that the compound will be transferred to the Swiss.