If Sol made a secret partnership with Barzini, his money would strengthen Barzini and ultimately embolden Barzini to make a move on Vito. Sol probably thought he'd have a real partnership with Barzini after Barzini moved Vito out of the Number One spot. That would be important to Sol, because he could not for long exist as a wealthy independent operator. He knew he'd have to throw in his lot with a powerful family. Vito, if he went for the deal, would operate at arms-length. Barzini could be a real partner.
Yes, he could have betrayed Barzini, but who would be left for him as a partner?
Sol only needed Vito's polital connections for protection. I'm not sure whether he needed help with actual street operations. He was a drug importer, not a dealer, and so he could sell his drugs to the "blacks" and other street gangs.
