Ali Asghar Zarean also denied reports Iran had dismantled the core of its Arak nuclear reactor, a key step in the deal that will see sanctions lifted in exchange for limits on Tehran's nuclear programme.
"Iran will sell 40 tonnes of its excess heavy water to the United States through a third country," Zarean, deputy head of Iran's Atomic Energy Organisation, was quoted as saying by the official IRNA news agency.
"Six tonnes of the exported heavy water will be used in nuclear facilities and the rest in American research centres," he said.
Iran has a heavy water production plant in its Arak nuclear site, which has been operating for several years.
Under its July deal with the P5+1 group -- the United States, Britain, China, France, Russia and Germany -- Iran has agreed to replace the core of the Arak heavy water reactor and take other steps to ensure it cannot be used to make nuclear weapons.
Signup to our newsletter and follow us on Facebook and Twitter!
Reports emerged on Monday that Iran had removed the core at Arak, but Zarean said this was not the case and that Tehran was still working on an agreement for a replacement being redesigned with the help of China and the United States.
"We must have a solid agreement with the foreign side, including China.... The documents of the agreement will be officially exchanged at the end of next week or this week," Zarean said.
"As long as the agreement is not finalised, we will not take any physical measures to remove the core of the Arak reactor."
Under the deal Tehran has reduced the number of its centrifuges and transferred the bulk of its low-enriched uranium stockpile to Russia.
The International Atomic Energy Agency (IAEA) must verify that Iran has fulfilled all of its obligations before sanctions can be lifted.
Officials have said the IAEA announcement could come as early as this month.