You need to get a grip on this because it’ll not be long before the child is bigger than you and stronger than you. Then they’ll start hitting you and you’ll be utterly bewildered about where it came from. DC with SEN can be taught other ways of dealing with issues in the moment, you need to start with that process as soon as possible.
They clearly don’t care about loosing money and yes, it’s like they can hit, pay the price and it’s all good. I’m amazed your older child hasn’t already hit them back.
My DC has complex needs and was a hitter. What worked was removing them from the situation every single time, no matter where we were or what they missed out on, I’d physically remove them if need be. That meant sitting with them at times, or getting them to come and do a chore or task with me - nothing fun or pleasurable but time next to me. If they wreck their joint bedroom, they work with you to put it back together again.
It also meant, when things had calmed down, helping them find ways to express their feelings and to reset their regulation system. That might mean going for a long walk, sensory input like water, play dough, making noise or music, dancing it out, throwing soft bean bags outside - anything that gets them moving, and brings them physically into their body. Cold drinks through straws (the sucking motion helps regulate). Basically you can’t discipline it out of them but you need to consistently address it.
With your older child id be clear it’s not ok, you’re working on it and show them that you’re working on it. If they can’t be together keep them separate, if that means one is in a room with you while the other does something else, so be it. Yes it’s bloody hard work but better now when you still have some influence than in a few years time when he’s tearing up your house.