Refunds on deleting prepaid billing period resources are calculated by refund formulas.
Example: You are using 2 dedicated IPs. As your account offers only one dedicated IP for free, you pay for the extra one. Removing one of them you will be refunded only if it is the IP that you prepaid for. If the removed IP was acquired for free, no recurrent fees are returned for the prepaid IP that appears to be free now. The system will treat this resource as free starting from the next billing period.
In such cases you are advised to remove the resource to get the refunds and then add it again as free.