Refunds are not possible, just like with EC2 RIs.
RI Billing Method
RI billing applies the reserved instance rate to all running DB instances if the number of running instances per hour is less than or equal to the number of reserved instances owned. Any usage beyond the purchased reserved instances is charged at the on-demand rate.
For example, if you own three reserved instances (or instance families with size flexibility) of the same database engine and instance type in a specific region, the billing system checks each hour how many running instances match those parameters.
If the count is three or fewer, each running instance during that hour is billed at the reserved instance rate. If it exceeds three, the excess instances are billed at the on-demand rate.
RI Size Flexibility
Amazon RDS Reserved Instances provide size flexibility for MySQL, MariaDB, PostgreSQL, and Amazon Aurora database engines, as well as Oracle BYOL (Bring Your Own License) editions. For size flexibility, the RI discount automatically applies to usage across all sizes within the same instance family using the same database engine. Size flexibility does not apply to Microsoft SQL Server or Oracle License Included (LI) editions.
For example, if you purchase a db.m5.2xlarge MySQL RI in the Seoul region, the RI discount automatically applies to two db.m5.xlarge MySQL instances. Also, the RI discount applies to usage in both single AZ and multi-AZ deployments of the same database engine and instance family.
For example, if you purchase a db.r3.large PostgreSQL single AZ RI in the Japan region, the RI discount automatically applies to 50% of the usage of db.r3.large PostgreSQL multi-AZ instances in the same region.