I don't think many drug dealers are the "pay me Tuesday" type...
Anyways, there's an obligation in justice to honor commercial transactions. I'm not sure exactly how absolute it is, but in this instance someone is paying for some type of good, and that good, though its subjectively intended use may not be lawful, is still a good with some objective lawful value (e.g., marijuana and cocaine were both common pharmaceuticals not just in American but worldwide before the twentieth century). We're not talking about paying a hitman here. So I don't think the fact that the drug dealer's supplier might harm him even has to enter into the equation. The dealer is owed the money simply by virtue of the transaction and the agreed upon contract.