The 65-year-old Social Democrat (SPD) challenger to Chancellor Angela Merkel, Steinbrück was one of the best earners in parliament, the Frankfurter Rundschau said on Tuesday.
His parliamentary website states that, from paid speeches to banks, companies and insurance firms, the former finance minister has been earning well over €500,000 since 2009.
Steinbrück was chosen by the SPD to lead the election campaign against Merkel's conservative Christian Democratic Union last week. The decision was made earlier than planned after pressure increased from the regions for a choice between a trio of SPD leaders to be made.
Horst Seehofer, Bavarian state premier and head of Merkel's sister party the Christian Social Union, called for Steinbrück to be transparent with his personal finances if that is what he demands from banks.
Steinbrück said he would not publish his income statements. “It is nobody's business how much I donate from the fees,” he said.
Ulrich Maurer, deputy leader of the far left Linke parliamentary fraction called on Tuesday for a law to force Steinbrück to open his accounts to the public.
Someone who is paid more than €7,000 several times for a speech is hardly paid for those efforts, he told the Leipziger Volkszeitung. He said only Steinbrück and the tax office knew at the moment who paid him how much and for what.
Yet Steinbrück said he had always carefully registered and paid tax on his income - publishing what needed to be published according to parliamentary rules.
The one seat Steinbrück said he was not going to give up while fighting to become Germany's leader was his seat on the supervisory board of Borussia Dortmund football club, which he said presented no conflict of interest.
Yet he said on Monday evening he was sorry that in 2006, while finance minister, he appealed to companies owned by the state to contribute millions of euros for a private chess tournament. Speaking on public broadcaster ARD, he said, “From today's standpoint, I would not do it again. But at that time I did not have the impression that it was dubious or dishonourable.”
He also said he could not step away from the raising of pension age from 65 to 67, despite the criticism of trade unions and left-wingers. “We will not be able to avoid the pressure from demographics... And in order to secure the financial basis of the social insurance system, we will need such changes,” he said.