Modify discount amount past appointment time to generate final receipt

I have an issue with invoice to make it a final receipt and include discount . I want to deduct the insurance claim amount from the original price. lets say I am giving a massage service for $100 an hour and the client has an insurance that covers 80%. when the client books initially, I dont know how much coverage they have and I will approve their appointment without this insurance coverage information. I was thinking to use the coupon discount option for this once service is provided. problem with this approach is that I can not edit and add the discount after appointment is approved and past appointment time. is there a way to add discount past appointment time just to generate a new invoice/receipt by including the discount which is based on the insurance claim?

lets say I have a client booked for massage service on May 20 from 2PM-3PM. service is $100. the invoice generated initially ($100) is okay if client can pay it but issue is that the client has partial coverage and need to reflect this in the receipt clearly. I can only include and deduct the insurance coverage once the client shows up and get the service which I will then collect insurance info and claim it. so how can I reflect this on receipt?

I was thinking to use the coupon discount option and create 80%, 20%, 40% discounts depending on customers insurance coverage but the issue is that I can not edit service to add discount amount once appointment time passed. Based on the above example of a massage service on May 20 for $100, I can go and add discount while status is pending/approved and before May 20 appointment time. the problem I am facing is that once appointment time passes, I can not edit and add that discount. with the above example, after providing service to the client on May 20, from 2PM-3PM, and if I get coverage amount from client, lets say clients coverage is 80%. if I try to edit and add a discount at 3PM for 80%. I can not edit and include discount and the edit option is no longer available once time passes? is there a way to keep it editable one more day or another option?