BaFin said the transaction took place on June 24, the day before Wirecard filed for insolvency.
It said it involved a 6.6 million euro ($7.52 million) sale in shares by MB Beteiligungsgesellschaft mbH. That is the investment vehicle of the former chief executive Markus Braun, once one of the collapsed payment company's largest shareholders.
A law firm representing Braun didn't immediately respond to requests for comment.
A further sale of 3.4 million euros in shares by the same investment company is also under investigation, a spokeswoman for BaFin said.
(Reporting by Hans Seidenstuecker and Alexander Huebner; writing by Tom Sims. Editing by Jane Merriman and Nick Macfie)
