@Randy_Cohen is correct, you never want to pour resin back into a cartridge (unless you don’t care about potentially contaminating the rest of the cartridge’s contents).
You may also want to check in on this thread once it gets a response from Formlabs:
There was some suggestion that emptying the tank may prolong its lifetime, if you’re not printing frequently. If you go that route, you’ll want to filter and store it in dark place in an opaque container.