Khashoggi, a Washington Post contributor and Saudi citizen residing in the United States, vanished on October 2 after visiting the building to obtain official documents.
US President Donald Trump has vowed "severe punishment" if proof of Saudi involvement in Khashoggi's disappearance is found.
Under Son, SoftBank built its global influence through tech investments and raised over $93 billion a year ago to create its Vision Fund - with $45 billion coming from Saudi Arabia.
Salman emphasised the importance of the Turkey-Saudi relationship and said no-one should be able to "undermine the strength of this relationship", official Saudi media said.
Saudi officials claim that the journalist left the embassy but Turkish officials say he may have been murdered inside the consulate by Saudi agents.
"The Kingdom affirms its total rejection of any threats and attempts to undermine it, whether by threatening to impose economic sanctions, using political pressures, or repeating false accusations that will not undermine the Kingdom and its staunch positions and Arab, Islamic and worldwide status, the outcome of these weak endeavours, like their predecessors, is a demise", the country's press agency said, citing an "official source".
Britain, France and Germany insisted Sunday that "light must be shed" on the whereabouts of journalist Jamal Khashoggi as they called for a credible investigation into his disappearance. Trump said there would be no U.S. sanctions on the oil-rich kingdom, claiming that the Saudi's were spending $110 billion (€95 billion) on USA military equipment.
The incident is threatening to spark a geopolitical crisis as the US and Turkey demand answers from Saudi Arabia, and the kingdom vows to retaliate against any punitive measures imposed over the critic's fate.
Doubts are also growing over whether British Trade Secretary Liam Fox will attend the Riyadh conference, the BBC reported Sunday citing diplomatic sources.
Few details have emerged over how the joint working group will function, sparking suspicion that Saudi Arabia might be trying to coax Turkey with certain concessions at a time of the latter's economic problems.
"We're going to get to the bottom of it and there will be severe punishment", he told CBS News.
Where is Jamal Khashoggi?... "It would lead to Saudi Arabia's failure to commit to producing 7.5 million barrels". In response, executives at some major USA companies announced they would no longer attend the "Davos in the Desert" conference hosted by the country. The Washington Post contributor, who was living in self-imposed exile in the U.S. since 2017, had reportedly gone to the consulate to collect some papers for his marriage.