I have a very nearly teen, who is in a school class (abroad, mixed age classes for various reasons) with 12-14 year olds so in a teen peer group, hangs out with teen friends etc. so I know I'm not "experienced" as a parent of teens, especially mid-iolder teens, but also not writing from the idealised point of view of the parent of preschoolers.
If I'd promised to pay for my eldest to go to the cinema, what she'd done with her Christmas money would not be relevant.
Unless there were strings attached to the offer to pay for a cinema ticket I'd still be paying.
The cannabis issue does cloud things massively. If this is really about the cannabis then I assume a lot more information is needed, but I do think I'd want my child to do as much socialising and getting out of the house where there definitely would not be any drug influence as possible.
If the cannabis issue is to be left out of the picture because it is not known whether this is what the money should be spent on, I don't think the Ds has actually done very much wrong, in spending his own money.
I also don't really see any reason to take back the offer to pay for him to go to the cinema.
The reason Sanshin is paying for him to go to see Star Wars is to make things equal with the fact his step dad already took his step brother, according to the opening post. Its nothing to do with Christmas money.