In addition the police think that the dealer does not know what he is selling. Heroin is much more expensive per gram than cocaine. And yes having seen both they would be pretty similar by sight. Especially in a little bag and poor lighting they would be remarkably similar. Given that the users are expecting cocaine, the dealer thinks he is selling cocaine and the drug is a white powder it is understandable that this is happening. Think Uma Thurman in Pulp Fiction.