British Iranian dual nationals Nazanin Zaghari-Ratcliffe and Anoosheh Ashoori could be freed and allowed to return to London within days under a deal in which the UK agrees to repay a £400m debt and release an Iranian prisoner. Zaghari-Ratcliffe has for months been at her parent’s home in Tehran awaiting news of whether she would have to serve a further one-year sentence in Iran handed down on top of the five years she had already served. Iranian diplomatic sources leaked news of the possible deal on Tuesday saying the UK had agreed to pay the debt, something London would only be doing if it had the agreement of the US to do so. The UK has previously said the debt payment could not be made because of sanctions on Iranian state banks. On Sunday, Zaghari-Ratcliffe was unexpectedly given her British and Iranian passports – a preliminary to her likely release. She has also been summoned to see officials at the ministry of intelligence and the Islamic Revolutionary Guards Corps to discuss her future behaviour. Confirmation of the return of her passport was published by Zaghari Ratcliffe’s MP, Tulip Siddiq. A UK government negotiating team is in Tehran. The prime minister, Boris Johnson, said discussions were delicate, adding it was best “you should say as little as possible unless and until the thing is actually concluded.” Ashoori, it is understood, has also been told to make preparations for his likely release from Evin jail, Tehran. It is likely they will be flown out of Iran via Oman. One source said: “This is the closest we have come to her release in six years, but I do not expect to see either of them at Heathrow in the next 24 hours.” The deal could yet unravel as it has in the past if there is a last-minute hitch – either over the talks on the Iran nuclear deal in Vienna or if the US insists other detainees are released. The Vienna talks should see a swath of US sanctions lifted on Iranian state bodies and industries, in return for Iran coming back into compliance with the constraints placed on its civil nuclear activities, including uranium enrichment. Russia threw a last minute curve ball at the Vienna talks on 5 March by demanding written guarantees that any Iran-Russian trade unleashed by the lifting of US sanctions must be exempted from the US sanctions imposed on Russia as a result of the invasion of Ukraine. Sergei Lavrov, the Russian foreign minister, met the Iranian foreign minister Hossein Amir-Abdollahian in Moscow on Tuesday to discuss Russia’s demands, and appeared to have backed down. He claimed to have received guarantees, but they do not appear to be the broad exemptions Russia has been seeking. He added it was a lie that Russia had ever wanted to delay the nuclear agreement. His remarks led oil prices to fall. Some officials said it is possible the two British-Iranian dual nationals will be released prior to the conclusion of the nuclear deal since the key bilateral Iranian demand of Britain has been the repayment of the £400m debt that has been outstanding since the 1970s. The UK, in court cases and letters to the Ratcliffe family, have acknowledged it owes the debt. Both Ashooori, sentenced to 10 years, and Zaghari-Ratcliffe, have always protested their innocence, with their families claiming they were being used as state hostages, or pawns in Iran’s demand for the repayment of the debt. The deal appears to exclude Morad Tahbaz, a British-American businessman born in Hammersmith, London, who also has Iranian citizenship. A conservationist, Tahbaz was sentenced along with eight others to 10 years in jail in November 2019 for “contacts with the US enemy government”. The British foreign secretary, Liz Truss, has tweeted frequently in support of his innocence, an acknowledgement that he is a British consular case. His family was in contact with both the foreign office and the US State Department on Tuesday, trying to find out if he had been excluded from any prisoner swap. A member of the Tahbaz family said: “I am asking the question ‘Why is it the only British-born UK citizen held hostage in Iran is being left out and abandoned? How is it that the UK is accepting Iran’s rejection of his nationality based on his birth?” She added: “In January 2021 the then foreign secretary, Dominic Raab, had previously sought our consent to get a sanctions waiver from the US for payment of the Chieftain tank funds in exchange for release of Ashoori and Zaghari-Ratcliffe whilst Morad would get a partial furlough and then be treated as a USA problem to secure his complete release. Morad in a message from jail refused such a deal and so did the USA since they expected the UK to work as hard for him for his permanent release.” The Tabhaz family say they had closely followed Foreign Office advice not to go public about his plight, but now felt betrayed since they had found out from the media what was happening behind their backs.