"In our invoices we mention a clause that buyers of our oil will have to pay in euros, considering the exchange rate versus the dollar around the time of delivery," the source said.
Lukoil and Total declined to comment, and Cepsa did not respond to a request to comment.
Iran has also told its trading partners who owe it billions of dollars that it wants to be paid in euros rather than US dollars.
The source explained that Iran’s central bank adopted a policy to carry out foreign trade in euros while the country was under sanctions.
"Iran shifted to the euro and cancelled trade in dollars because of political reasons," the source underscored.
According to US officials, nearly $100 billion of Iran’s assets were frozen abroad, and around 50 percent of them Tehran was able access after the removal of sanctions.
Sanctions against Iran were lifted on January 16, by both the US and the European Union. The move came amid oversupply in the global oil market, with the Organization of the Petroleum Exporting Countries (OPEC) refusing to cut down its daily output.
It is not clear how much of the funds are oil dues that Iran wants back in euros. For instance, India owes Tehran around $6 billion for oil supplied during the sanctions years.
For many years, Iran has pushed to replace dollar with euro in international oil trade. In 2007, Tehran failed to persuade OPEC to switch away from the US dollar which then-president President Mahmoud Ahmadinejad called a "worthless piece of paper."
Switching oil sales to the euro makes sense for Iran since Europe is its major trade partner. In late January, The Wall Street Journal reported that Iran had agreed to lower price of its crude for Europe, following a similar move by Saudi Arabia. At the same time, Tehran increased crude prices for Asia.
