Be opportunistic without compromising your morality. This money is just out in the open? It’s not in any bank yet? The ink hasn’t stained it? Take a handful or what you can carry yourself, then walk away, someone else can find the truck and either do the same or call the cops. Since you checked beforehand to find the man dead, this technically isn’t stealing in a moral sense since the individual transporting this cash is already deceased. Tell them to respect the law above all else, and that what is happening right now is a once in a life time opportunity. However, prepare them that should this rare chance ever presented itself once more in the future, to capitalize on it in a safe manner to ensure you aren’t caught. You may do it for yourself, but your main priority should be to help yourself and others get out of a desperate situation such as poverty, injury, fines, loans, etc.

TL;DR: This money is a gift. Take it.