Truman had estimated that he could get "85 percent" of his way with Stalin prior to their first and only meeting at the Potsdam Conference. He believed the Soviet foreign minister Molotov - who he had earlier "told of" - to be the problem. I think that as negotiations at Potsdam showed that Stalin had his own ideas about how the postwar World should be formed, Truman got increasingly displeased with the course of the conference. He left it more and more to his newly appointed foreign secretary Byrnes to handle the negotiations.
When Truman was informed of the succesful testing of the Manhattan project during the conference, it livened him up. He believed that the Soviets would be more compliant when they saw the result of the bomb. He even told Stalin about it, but the Soviet premier didn't react as Truman thought he would. It has later been established that Stalin had been informed of the bomb by Soviet spies.
The Soviets had promised at the Yalta Conference that they would invade the japanese positions in Manchuria, and not go against the nationalist Chiang Kai-shek in China, who fought the chinese communists at this time. During the beginning of the Potsdam Conference Truman was very interested in getting Stalin to commit to these promises. However, after the bomb had been tested, Truman and Byrnes seemed to think that the war against Japan could be ended before the russians entered, and thus eliminate any influence the russians could get over Japan, which indeed became the end result.
So the argument that the bomb was used to "send a message" to Stalin is a valid claim. However, I do not think that it can stand alone. For the reasons given in my previous post, I still think that Truman not using the bomb would be unthinkable. It certainly didn't discourage him that he could send a message to Stalin, but it was not the only reason to use it, and in my opinion, it was not the most important reason.