On a yacht moored at Saint-Tropez, Malaysian Prime Minister Najib Razak talked business with Abu Dhabi’s crown prince. Included in the horseshoe of chairs set up for the July 2013 gathering was a partner from Goldman Sachs Group Inc.

The bank earned its place through years of cultivating Mr. Najib and a state investment fund he founded. Goldman had raised $6.5 billion for the fund and earned nearly $600 million in fees, making the Malaysian client among its most lucrative.

Mr....