Belgian MPs ratified a controversial treaty with Iran that could allow an Iranian convicted of terrorism in Belgium to be sent back to Tehran.

Out of the 131 MPs present, 79 voted in favor while 41 rejected the treaty, and 11 abstained from the vote just after midnight, following a late debate Wednesday.

Critics fear that the treaty, which would permit Iranians convicted in Belgium to serve their sentences in Iran and vice versa, will pave the way for the release of Assadollah Assadi, a terrorist convicted in the Belgian courts and sentenced to 20 years in prison for his role in a bomb plot targeting a rally by opponents of the Iranian regime in France.

The treaty also allows each party to grant amnesty, and there is little doubt that Assadi, who worked as an Iranian diplomat, would quickly be set free.

Tehran meanwhile has been holding Belgian humanitarian worker, Olivier Vandecasteele, in jail since February as leverage.

The treaty was approved by the parliament’s foreign affairs committee on July 6, which led to heated debate among MPs, with even some members from the ruling coalition critical of the accord. A few of their lawmakers shared concerns about why the treaty was rushed through parliament before the summer break, and said they weren’t able to consult enough external experts ahead of the vote.

“We had to be able to publicly express our reservations and deep concerns, and at the same time do our job as parliamentarians,” said Samuel Cogolati, a Green MP who is part of the governing coalition, ahead of the vote. “We tried as much as possible to consolidate, to give guarantees for the rule of law, while ensuring the release of our Belgian compatriot in Iran.”

The signoff by the full parliament was almost a given after the committee’s approval.

Last week, Belgian Prime Minister Alexander De Croo responded to MPs who denounced the deal as “blackmail” and a form of paying ransom to Iran for Vandecasteele’s release.

“What do you say to his family, that we will let him rot in his cell?” said De Croo. “Belgium does not abandon its citizens.”

Vandecasteele’s family has been urging Belgian authorities to take steps to release him, and welcomed the vote. “We as a family are proud to see [that] some politicians are taking the right decision to free an innocent man in horrible conditions in jail,” said Olivier Van Steirtegem, who is representing the family.

“It’s never a fair trade with a terrorist, but it’s also not fair to keep an innocent man in jail,” he added.

Reacting to the vote, François De Smet, the leader of the opposition party Défi, said the treaty would set a precedent that “will have terrible repercussions on the reputation of our country and the safety of our citizens.”

“Belgium sends the message that its justice is for sale,” he tweeted.