He also met Crown Prince Mohammad bin Salman (MbS) who offered him his condolences for his father's death.
CIA Director Gina Haspel on Thursday briefed Trump on the evidence about Khashoggi's death that she reviewed during a trip to Turkey.
"It is tragic that it took his death to get the Saudi authorities to grant them the freedom to travel", Slim told AFP.
However, even US President Donald Trump has cast doubt on MBS's claim he had nothing to do with Khashoggi's murder.
Turkish media have also published a security camera image allegedly showing a vehicle belonging to the Saudi Consulate "scouting" a forest in the outskirts of Istanbul before Khashoggi was killed.
Khashoggi - a Washington Post contributor and critic of the crown prince - was killed after entering the Saudi consulate in Istanbul on October 2.
"We have shared with those who sought additional information some of the information and findings that the prosecutor has allowed us to share", Turkish Foreign Minister Mevlut Cavusoglu told reporters, without giving details.
"We have maintained since the beginning that the Khashoggi murder was premeditated", a senior Turkish official told The Washington Post shortly after the Saudi announcement on Thursday.
"Whether or not they acted in the name of the state, that remains to be discussed and investigated", she said.
After weeks of denials, the kingdom finally confirmed Khashoggi's death on October 20, claiming he died in a fistfight.
"The chancellor urged Saudi Arabia to ensure a rapid, transparent and credible investigation". The Saudi royal family has denied involvement, instead scapegoating a number of Saudi agents who they say are responsible for the murder.
She said: "Good news for a change: confirming that Jamal Khashoggi's son Salah and his family are finally out of Riyadh and on their way to the United States, travel ban lifted".
One, his hands painted red, wore a mask depicting the face of Saudi Arabia's de facto ruler Crown Prince Mohammed bin Salman.
Earlier on Wednesday, a close aide to Erdogan accused Prince Mohammed of having "blood on his hands".
The king assured Russian President Vladimir Putin in a phone call on Thursday that the Saudi government was determined to hold the guilty parties accountable and make sure "they receive their punishment", according to the official Saudi press agency.
The affair prompted numerous high-profile political and business leaders to pull out of Saudi Arabia's Future Investment Forum (FII) in the capital Riyadh this week.
"We will prove to the world that the two governments (Saudi and Turkish) are cooperating to punish any criminal, any culprit, and at the end justice will prevail", he said to applause.