Shopify API - Apply Discount on already created order

Shopify Partner
11 0 1

Hey Guys!

Is it possible to apply a discount on an already created COD order and mark it as Paid, using Shopify APIs?

Example: Order was created with a total of $100 Pending Payment with COD mode. Now, I want to give a discount of $20 and mark that order as Paid.

Problem: If I only charge $80 right now, Shopify marks it as Partially Paid order, which is correct. But I need to show $80 as Paid and $20 as Discount and Order marked as Paid.

Please help. Thanks in advance.