He must also reassure investors and credit-rating companies of credible plans to stabilise the debt and revive growth in the mid-term budget on October 24.
"Its a good appointment, especially if you want to appease the markets", said Mcebisi Ndletyana, a political science professor at the University of Johannesburg. "He will strengthen Ramaphosa's hand in cabinet".
In a move that was cheered by markets, the former Reserve Bank governor returned to the cabinet for the first time since 1998, when former president Nelson Mandela deployed him to the Bank, before he took over after Chris Stals retired in 1999. His major achievement at the central bank was building the nation's foreign-exchange reserves to nearly US$40-billion from less than $10-billion. He is urging those ministers and other senior officials to follow Mr. Nene's example by volunteering to give testimony at the national inquiry into corruption so that their conduct can be reviewed. He featured on a list of candidates for the panel that backed Nkosazana Dlamini-Zuma for the position of party leader in December - a post won by Ramaphosa.
Nene admitted to meeting the controversial Gupta family at their Saxonwold residence at the state capture commission of inquiry last week.
Ramaphosa said he had received a resignation letter from Nene and had chose to accept it. Nene did not answer calls for comment. It is a measure of his character that he had taken this decision.
Nene was seen as one of Ramaphosa's closest allies and a leading figure in the government's efforts to tackle graft that allegedly flourished under former president Jacob Zuma, who was ousted in February.
This was Nene's second stint as finance minister.
But he had accepted the resignation because Mr Nene feared his testimony "detracted from the important task of serving the people of South Africa as we work to re-establish public trust in government", Mr Ramaphosa said.
Ramaphosa has made clean governance and the kick-starting of an economy mired in recession top priorities. Jonas, Mboweni and Kganyago didn't answer calls to their mobile phones.
With government debt at already dangerously high levels and the country at risk of losing its sole remaining investment-grade credit rating from Moody's Investors Service, Mboweni will have little option but to stick to the National Treasury's expenditure ceilings and deficit-reduction targets.