The Nigerian president has also asked the U.S. for help in reclaiming billions of dollars of stolen funds.
The trials of a pro-Buhari lobby group have sparked a social media storm.
Ex-Nigerian ruler Sani Abacha is suspected to have looted up to $5 billion in public funds during his reign.
The nonexistent workers were removed from the state payroll following an audit launched in December.
Some 20 people died in stampedes at a recruitment drive by Nigeria's immigration service in 2014.
Allegations against Saraki include that he acquired wealth beyond his legitimate earnings.
A recent report claims that Nigeria's leader has only fulfilled one of 222 electoral promises in seven months.
Muhammadu Buhari is cracking down on corruption in the Nigerian government.
President Buhari has ordered that 20 retired and serving military officials be investigated for corruption.
Information minister Lai Mohamed said ex-ministers and bankers were among those responsible.
The IMF managing director is visiting Nigeria as the country struggles to deal with a global oil slump.
Corruption is part of life in sub-Saharan Africa, with faith in the police and courts at rock bottom.
Sambo Dasuki allegedly awarded phantom contracts for arms to fight the Nigerian militant group.
The newly-elected Nigerian president says he will restore financial "sanity" back to the country