Financial prosecutors Grigoris Peponis and Spyros Mouzakitis have asked the economic police unit to investigate the deposit for possible unlawful activity, the Athens News Agency reported.
Also, according to information reaching the prosecutors, Djebbour has allegedly not submitted a tax return since 2007 despite being obliged by law.
On Monday the 28-year-old said he had agreed to extend his contract with the Greek champion for the next three years.
He made the announcement on Olympiacos' website and after agreeing with team boss Vangelis Marinakis.
"Money was never my priority and I want all of the Olympiacos family to know that I never wanted to leave Piraeus and will do everything to have our fans happy and to celebrate with them and my team-mates victories and titles," Djebbour said.
But Greek press reported on Thursday cthat Djebbour may be sought by English champion Manchester City.
Djebbour was a member of the Algeria team at the 2010 FIFA World Cup and has 28 caps, scoring five goals for the national side.