[anabis]

You are right, my mistake.

Although it seems hard to press charges without the property owner's cooperation.

Can express permission to paint a graffiti given afterward?

[jasomill]

Surely not. Better question: is "vandalism" without criminal trespass or property damage actually a crime? If it's on private property, and the property owner enjoys the painting, it seems like it'd be quite a stretch for the state to establish that the artist damaged the property by creating it.

[yetanotherphd]

You could always lie and say you gave him permission, but that would probably be a crime in itself.