How to get the last value of the last paid date and leave it constant for the subsequent months?

[#1]

Good Morning

hi, I need help,

I have to show the sales of products from a store, but this store gives its clients facilities to pay a certain period, for example a pantalon costs 650 and they give us a time to pay the payment every month, then there is a field where Called "Useful Life of Payment" where in that field goes the time in which the pants will be paid, if the pants were bought on the date 01/02/2012 and they tell me that I have a useful life to pay of 5 Years, then the year-end date must end on 02/01/2017. What I need to do is obtain the sum of the field "Value" but still continue considering the last value of the date of my last payment, for the months of 01/03 / 2017,01 / 04/2017 ... 01 / 12/2017. That all my products already finished to pay that their final value settled will continue to be considered for the remaining months until December 2017. only that the value of the last payment for the year in which it was paid will be considered, for example if the pants were liquid 01/04/2017 only consider the last value for the year 2017, after 2017 that no longer considers the value settled for 2018.

I leave an image to understand better, in the table on the right side I identify the yellow column means that this is the last payment date and the columns painted green means that they should consider the last value paid for the dates of the painted fields of green.

I hope my problem understood

thank you very much