Refund or Stop Paying Installments if You Quit Invisalign Mid-treatment?

Your treatment plan was a contract between you and your dentist to exchange an agreed amount of money for completed treatment. Your dentist has already paid Invisalign for the entire treatment and done the majority of the work even though you are only half way through treatment.
Do the right thing and finish the tereatment and pay the entire amount! Your dentist was nice enough to let you make payments. I get paid upfront for invisalign or the patient enters into a financial agreement with a 3rd party that charges interest.

In MY office, the fee is for the result.  If you are happy with what you have now, there would be no refund at all.
I would talk to your dentist or orthodontist to see what they think, but I would not expect a change in fee or refund.  The costs have already accrued and the aligners have been made, whether you decide to use them or not.
It would be like buying groceries to feed a large group.  If fewer people showed up, the costs didn't change.  All of the aligners have been created, so even though you haven't used them all, the dentist has already paid for them.