From wiki

Greene had requested Birns for a loan of $75,000. Greene wanted the money to set up a "cheat spot," a speakeasy and gambling house. Therefore, Birns had arranged a loan for Greene through the Gambino family. Somehow later, the money wound up in the hands of Billy Cox, a numbers operator, who used it to purchase narcotics. The police raided his house, arrested him, seized the narcotics and what was left of the $75,000. The Gambino family, from whom Birns had borrowed the loan, wanted their money. Birns pressed Greene but Greene flatly refused to return the money; instead asserting that it wasn't his fault that it got lost.

Soon after Birns gave 25000 $ to an hitman for kill Greene but all attempt was unsuccessfull and Greene made a bomb that killed Birns.