The Social Democratic leader will be heard from 12:00 GMT by the Hamburg Parliament's commission of inquiry into this complex fraud revealed in 2017.
The CumEx scandal relates to an ingenious system of tax optimization set up by banks and allowing foreign investors to reduce their taxes on dividends.
Dozens of people have been charged in the case in Germany, including bankers, traders, lawyers and financial advisers. A total of ten countries are concerned.
- Entering emails -
Among the banks incriminated is the Warburg in Hamburg, which should have reimbursed 47 million euros to the German port city, but the municipality had waived it in 2016.
The bank finally had to pay tens of millions of euros in reimbursement under pressure from the government of Angela Merkel.
Investigators are trying to find out if political leaders - and among them Olaf Scholz, then mayor of the city - pressured the municipal tax authorities to stop collecting these taxes.
"I can't tell you anything about it," government spokesman Steffen Hebestreit kicked in on Wednesday, assuring that Mr. Scholz would "answer" all the committee's questions.
The decision to waive reimbursement of amounts owed by the Warburg bank was reportedly taken shortly after a conversation between Olaf Scholz and Christian Olearius, then head of the bank.
Mr Scholz, however, denies lobbying tax officials in the city of Hamburg, which he led between 2011 and 2018.
But new elements revealed in recent days undermine the denials of Angela Merkel's successor.
Several media have revealed in recent days that emails from someone close to Mr. Scholz had been seized by investigators and would provide "potentially conclusive" information about "thoughts on the deletion of data".
This email "overwhelms" Mr. Scholz, summarizes the daily Frankfurter Allgemeine Zeitung.
At another elected SPD, likely to have played a role in the reimbursement by the bank, the investigators discovered more than 200,000 euros in cash in a safe, fueling suspicions about possible hidden financial arrangements.
- Gaffes -
Other documents seized would also suggest that Mr. Scholz, contrary to what he has said so far, would have raised the subject of reimbursement directly with Christian Olearius.
The opposition has thus rushed into the breach to criticize the leader of Europe's leading economic power.
"The latest revelations suggest that Olaf Scholz and his direct entourage have endeavored to give only limited information on certain meetings or telephone conversations (...) or to conceal them voluntarily in order to protect the current Chancellor", denounces such as Matthias Hauer, conservative member of the Bundestag Finance Committee. These revelations are, according to him, "politically explosive".
They fall particularly badly for Olaf Scholz, who has become less popular than many of his ministers, less than a year after taking office.
The ex-mayor of Hamburg is indeed struggling to reassure the Germans, worried about possible energy shortages this winter, in particular of gas, in the context of the Russian invasion of Ukraine. Europe's economic engine is therefore at risk of entering recession in the coming months.
Gaffes also weaken the chancellor. On Tuesday, for example, he did not react immediately to the remarks of the head of the Palestinian Authority, Mahmoud Abbas, who compared in Berlin during a joint press conference the Israeli policy towards the Palestinians to the Jewish genocide by the Nazis. A lack of responsiveness that attracted a volley of criticism.