Most "dealers" are actually charged with 'intent to distribute', a crime that doesn't require evidence of money changing hands. Giving drugs to your friends, even for free, will result in basically the same drug charges as if you were selling. The law is setup this was specifically to avoid the need to money evidence. The cops only need to see you hand over the drugs. They don't need to also see you receive any money. (Actually, you don't even need to hand over the drugs. Possess a large amount, or a smaller amount divided into little bags, and that will be enough for intent to distribute.)

Many street-level drug dealers will still separate the drug and money handovers between different people. This isn't to avoid legal issues. It is to avoid the money being seized when the cops move on the guy they see handing over bags of drugs.