I inferred that Sollozzo hadn't volunteered that he was giving a share to the Tattaglias. But he should have known that Vito and Tom would figure that the Tattaglias would have to get something from Sollozzo in return for being his partners. (In the novel, Hagen tells Vito that Sollozzo "is vouched for by the Tattaglias.) Maybe Sollozzo was just buttering up Tom.