Ilych Ivankov and Lavro Kessar, as well as a bodyguard for each men met with Sark in Moscow discussing the terms of a Rambaldi artifact. After Ivankov said he wouldn't sell the artifact, despite being offered $100 million, one of Sark's bodyguard shot and killed Ivankov and his guard. Lavro and his guard quickly put their hands up in protest, not wanting to be killed. Sark told Lavro that he was now the new leader of the group and that he had 30 seconds left to agree, which he did (The Coup).
After Ivankov's murder, Lavro went missing and was presumed captured by Sark and "The Man." However, Lavro showed up a few days later, apparently still held captive by Sark. Sark traveled to Tunisia to collect the Rambaldi manuscript in exchange for Kessar's life, but he never received it. Sydney had subdued all the K-Directorate agents on the yacht the exchange was supposed to take place and stole the boat, along with the manuscript (Page 47).
Since Sark never received the manuscript, it can be assumed that he stayed true to his word and killed Kessar. However, since Kessar's body was never found, and since he was never mentioned shortly after the botched trade, it's possible that Sark was just bluffing.