Yes you can brown meat the night before slow cooking it.
After you brown the meat the meat just place the browned meat into a container and refrigerate the browned meat.
However if you're slow cooking other food and adding the browned meat to the slow cooker then you can just go ahead and start the slow cooker that night.
Then brown the meat the night before and go ahead and just put the browned meat into the slow cooker and then the food will be slow cooked over night and ready to serve or eat the next day.
That's the great thing about slow cookers.
You can slow cook food while you sleep which is what I've done myself for years.