There are many ways to pay for the service you might wish to purchase. If you are an active employee, your payment options are based on the amount of your buyback. If you have left your employer, please see the Buyback Proposal for your payment options.
You may have a set amount of money you want to pay towards your buyback, or perhaps your finances change before you are finished paying, and you decide you no longer wish to make payments. You can pay for a portion of the service you wish to buy instead of the full amount. This is sometimes called prorating your service.
For example, if the cost of buying one year of service was $10,000 and you decided to stop making payments after paying $5,000, you would receive credit for about half a year's service (before any interest charges get factored in).