Grenke on Friday issued its most extensive rebuttal yet to a 64-page report by the short-seller, Viceroy Research, which was released on Tuesday and which accused the company of malfeasance, causing its shares to dive.
It was Viceroy that raised the alarm and bet big against Wirecard, once a German success story and later a major embarrassment for politicians and regulators.
"We are honourable businessmen, and we refute any comparison with Wirecard," said the company's chief executive, Antje Leminsky.
Leminsky, speaking on calls to journalists and analysts, said that the special review by its current auditor KPMG would be concluded as quickly as possible. KPMG, which was also criticised in the Viceroy report, declined to comment.
Germany's financial market watchdog, BaFin, the nation's accounting oversight body, and prosecutors in Mannheim said they were investigating the Grenke case in the wake of the allegations.
A central allegation of the Viceroy report is that the company over the years bought various franchises without disclosing that corporate insiders, including founder Wolfgang Grenke, already controlled those franchises and stood to profit from the transaction.
In a statement on Thursday, the company said that, until January, Wolfgang Grenke did not own or have say over any of the shares of the vehicle that controlled the franchises, and he received no payments.
The company on Friday declined to answer questions about who previously owned the vehicle, CTP, though the CEO said "we have nothing to hide".
Ratings agency S&P has put Grenke's credit rating on negative credit watch. The company's shares lost half their value earlier this week, dropping from around 55 euros a share to as low as 24 euros, before recovering on Friday to close at just over 33 euros a share.
Union Investment, a top investor, said it was "checking the validity of the allegations" and would then "make adjustments to the fund if necessary."
(Reporting by Tom Sims, Patricia Uhlig and John O'Donnell; Editing by Ludwig Burger, David Evans and Louise Heavens)