SARAJEVO: An expected POW exchange was put on hold when Bosnia's Muslim government balked at the last minute, delaying the planned release of more than 500 Serb prisoners of war. The move cast new doubts on the future of the Dayton peace agreement, which calls for all sides to release their prisoners by Friday. Bosnian government officals say that Serbs have not lived up to their end of the bargain by offering only to release 200 Muslim prisoners. At issue for the government are the some 24,000 Muslims unaccounted for in 3 1/2 years of Balkan war. Many are believed to have been executed by Serbs and burried in mass graves near Srebrenica and the Lucovica mines, and Muslim officials hope to use Serb prisoners as a bargaining chip to learn the fate of those missing.