The device payment will never decrease.  The monthly price is for 24 months.  A credit, if you re-word it and move that amount to the end of the 24 months, if it can be held onto that long, would save that amount, but as far as I know, the credit is used first on an account for the bill.  This keeps customers from taking the credit and paying off the phone before 24 months, thereby reducing the time of service and income for Verizon.  Ex:  $300 credit is $300...but the form of credit, bill, card, or straight out, changes the use of the credit.  I for one would, if possible, leave the $300 on the account until I owed that amount on the device, then pay it off, but most people can't afford to let $300 sit unused.