Finance Minister Nhlanhla Nene is expected to testify before the judicial commission of inquiry into allegations of state capture on Wednesday.
Nene is set to take the stand at 10:00.
He is expected to reveal how he was "pressured" by former president Jacob Zuma to approve a nuclear deal. His refusal is believed to have been the trigger for his removal.
In August, former deputy minister of finance Mcebisi Jonas told the commission that Nene was dismissed by Zuma in December 2015 because he did not want to authorise the deal.
Zuma was adamant that the country should enter into an agreement with Russia for the construction of nuclear power stations.
According to Jonas, a man he believed to be Ajay Gupta offered him Nene's job if he agreed to help the family increase their earnings from the fiscus through various entities, including Eskom, Transnet and government departments, from R6bn to R8bn.
"Mr Gupta said they had determined that the National Treasury was the stumbling block for their growth and that they wanted to clean up Treasury," Jonas said at the time.
The meeting was held weeks before Nene was fired on December 9, 2015, and replaced him with Des van Rooyen, who only lasted the weekend as Zuma was forced to replace him with Pravin Gordhan, who returned for a second stint as minister.
Nene's firing sent the economy spiralling and caused widespread outrage from the ANC, business, civil society and ordinary South Africans.