Alexei Navalny was about to be freed in a prisoner swap when he died, according to his ally Maria Pevchikh.
She said the Russian opposition leader was going to be exchanged for Vadim Krasikov, a Russian hitman who is serving a life sentence for murder in Germany.
Two US citizens currently held in Russia were also going to be part of the deal, Ms Pevchikh claimed.
She added that negotiations were at their final stage on 15 February.
The next day, Mr Navalny died in his cell in the prison colony in Siberia where he was being held on a 19-year sentence over charges that were widely seen as politically motivated. Prison officials said the 47-year-old had fallen ill following a "walk".
In a video posted on Mr Navalny's YouTube channel, Ms Pevchikh, who is the chairwoman of his Anti-Corruption Foundation (FBK), said negotiations for a prisoner swap had been under way for two years.
She added that after the start of Russia's full-scale invasion of Ukraine in February 2022 "it was clear that Putin would stop at nothing" and that Mr Navalny "had to be freed from jail at any cost, and urgently".
According to Ms Pevchik, Mr Navalny was going to be freed under a humanitarian exchange and American and German officials were involved in the talks.
The process finally resulted in a concrete plan for a prisoner swap in December, she said.
Vadim Krasikov - a Russian who was found guilty of shooting former Chechen rebel commander Zelimkhan Khangoshvili in the head at close range in Germany in 2019 - was going to be part of the deal.
Two US nationals currently held in Russia were also going to be exchanged, Ms Pevchikh said, although she did not name them.
However, earlier in February, President Putin told US host Tucker Carlson that talks were ongoing with the US about freeing American journalist Evan Gershkovich, who is being held on espionage charges.
President Putin hinted that in exchange Russia would accept a person who "due to patriotic sentiments, eliminated a bandit in one of the European capitals… during the events in the Caucasus" - almost certainly a reference to Krasikov.
According to Ms Pevchikh, Russian President Vladimir Putin changed his mind about the deal at the last minute. She said he "could not tolerate Navalny being free" - and since there was an agreement "in principle" for Krasikov's freeing, Mr Putin decided to "just get rid of the bargaining chip" and "offer someone else when the time comes."
"Putin has gone mad with hatred for Navalny," Ms Pevchikh said. "He knows Navalny could've defeated him."
As a former KGB officer, President Putin is used to saying - or promising - one thing, and then doing something completely different.
It is a policy he and his government have been consistently implementing for almost a quarter of a century.
Up until the day Russia launched its full-scale invasion of Ukraine, President Putin and several Russian officials repeatedly denied there was a plan to invade the country.
Although we do not know what exactly happened to Navalny in prison, engaging in negotiations on his release without intending to set him free would fit the Kremlin's behaviour over the past years.
Within an hour of publication, Ms Pevchikh's video had had hundreds of thousands of views.
The Kremlin has not yet reacted to the claims put forward by Ms Pevchikh, but President Putin's spokesman Dmitry Peskov has previously said allegations of government involvement into Navalny's death were "absurd".
Authorities initially refused to hand Navalny's body over to his mother, only relenting eight days after his death.
On Monday, Navalny's spokeswoman Kira Yarmish posted a message on social media saying his allies were looking for a venue where supporters could hold a public farewell later this week.
Such an event is expected to be closely monitored by the authorities, provided it is allowed to go ahead at all.
A rights group said 400 Russians were arrested across the country for laying flower tributes to Navalny following his death.
Source: BBC
