> Rather than trying to do this in a single table, why not have a parent table of purchases, with a child table of payments for the purchase, each record of the child table containing the fields needed; a parent/child form with a grid for payment details would seem to solve the problem neatly.

Am I able to shown a list of due dates in the grid, when the user enter the numbers of instalments, and taking the number I have to show on the grid the numbers of due dates?