In the spring of 1943, Feldhendler led a small group of Sobibor prisoners in formulating an escape plan. Their initial idea had been to poison camp guards and seize their weapons, but the SS discovered the poison and shot five Jews in retaliation. Other plans included setting the camp on fire and escaping in the resulting confusion, but the mining of the camp perimeter by the SS in the summer of 1943 rendered the plan impractical. In late September 1943, a Holocaust transport of Jews from the Minsk Ghetto arrived. Among them was a Soviet POWs officer of the Red Army, Alexander Pechersky, who survived the selection to gas chambers. His presence gave new impetus to the escape plans. Pechersky soon assumed the leadership of the group of would-be escapees and, with Feldhendler as his deputy, devised a plan that involved killing the camp's SS personnel, sending the remaining Soviet POWs to raid the arsenal and then fighting their way out the camp's front gate. The uprising, which took place on 14 October 1943, was detected in its early stages after a guard discovered the body of an SS officer killed by the prisoners. Nevertheless, about 320 Jews managed to make it outside of the camp in the ensuing melee. Eighty were killed in the escape and immediate aftermath. 170 were soon recaptured and killed, as were all the remaining inhabitants of the camp who had chosen to stay. Some escapees joined the partisans. Of these, ninety died in combat. Sixty-two Jews from Sobibor survived the war, including nine who had escaped earlier.
Feldhendler was among those who survived the war, hiding in Lublin until the end of German occupation. The city was taken by the Soviet Red Army on 24 July 1944 and became the temporary headquarters of the Soviet-controlled communist Polish Committee of National Liberation established by Joseph Stalin. However, on 2 April 1945, Feldhendler was shot through the closed door of his flat as he got up to investigate a commotion in an outer room. Feldhendler and his wife managed to escape through another door and made their way to Lublin's Św. Wincentego á Paulo hospital, where he underwent surgery, but died four days later. According to most of the older publications, Feldhendler was killed by right-wing Polish nationalists, sometimes identified as the Narodowe Siły Zbrojne, an anti-Communist and anti-Semitic partisan unit. However, more recent inquiries, citing the incomplete treatment of the event by earlier historians, and the scant documentary record, have called into question this version of events. The only concrete document found by local Polish scholars is a record of Feldhendler's hospital admission at Wincentego á Paulo describing the injury. Dr Kopciowski wrote that Feldhendler was likely shot in an armed robbery gone bad, because he was known locally as a budding gold trader. Meanwhile, as noted by Marcin Wroński, communist press in the Soviet-controlled Lublin routinely accused former AK and WIN partisans of common crime as part of ideological warfare. Feldhendler's killing was one of at least 118 violent deaths of Jews in the Lublin district between the summer of 1944 and the fall of 1946.
In the 1987 made-for-TV film Escape from Sobibor he was played by Alan Arkin. Feldhendler's life in Lublin is mentioned in the 2005 book Wyjątkowo długa linia by Hanna Krall. It was written about tenants of a local tenement house, and nominated for the Nike Award.
