The release of Nazanin Zaghari-Ratcliffe, the British-Iranian woman who has been incarcerated in a Tehran prison since 2016, could come a step closer when a long-standing dispute over non-payment of a 400 million appeals court this week.

In a tweet, the Iranian ambassador to the UK claimed that a decade-long dispute between the UK and Iran over the size of the debt could soon end, leaving the government with no excuse for delaying payments. The prediction could prove optimistic given the legal complexities surrounding paying for the tank sale and the legal restrictions preventing the UK from circumventing sanctions on Iran.

Zaghari-Ratcliffe’s husband, Richard Ratcliffe, is due to meet Boris Johnson on Thursday. Zaghari-Ratcliffe was sentenced to five years in prison in 2016 on charges of espionage, which she has denied.

The dispute concerns an Iranian payment in the 1970s for 1,500 Chieftain tanks and armored vehicles. The contracts were canceled after the Shah was ousted in the 1979 revolution, but Iran had already paid for the undelivered tanks and demanded its money back. The two sides have argued in court over the amount of debt, including whether to pay interest.

Despite strong official refusals on both sides, the UK’s failure to pay the debt is seen as an obstacle to Zaghari-Ratcliffe’s release.

Iranian Ambassador to Britain, Hamid Baeidinejad, tweeted, “The Court of Appeal is due to settle a small portion of the outstanding debt.” He added, “By assigning this small portion, the legal process of the case will end and there will be no apology for late payment . “

The appeals court is due to hold a two-day hearing on Wednesday and Thursday on whether to pay interest on the debt. Last year, the Supreme Court in London ruled that the UK does not have to pay interest on the debt due to EU sanctions laws.

The UK has provided nearly £ 500m in surety for the debt. The International Military Service, then a subsidiary of the Department of Defense, said Britain would not be able to pay even if both sides agreed on the amount of debt because the Iranian Defense Department has been subject to EU sanctions since 2008.

Prior to Ratcliffe’s meeting with the Prime Minister, the British Ambassador to Iran, Robert Macaire, was to meet him and other British families of people detained in Tehran as part of regular updates.

The appeal court hearing was highlighted in the Iranian pro-government press this week, suggesting the case is expected to near closure. Iran has called for the debt to be paid to its central bank, but the UK appears to be resisting it.

Defense Minister Ben Wallace, who is also an expert on Iran, is said to have looked for alternatives to debt settlement, including through humanitarian aid such as food and medicine. Iranian officials have been trying to see if the UK sees this as a serious option and some senior officials could visit London for talks.