If the total price of the multipack is manually edited or a promotion is applied, the amount of the instalments that have been pre-configured for the multipack during the planning will not be affected by the promotion. It will be necessary to change the amount manually by clicking on the white icon next to each instalment in the corresponding section of the user profile. 
In case of fixed promotions, it will be possible to create a price list with two rows, one full price and one with the discounted price. In this case, the instalments will have to be created in the details of each row of the price list (please note that it is not possible to create instalments while creating a price-list row. Just create the row and save it and then click on Detail next to the price-list row to create the instalments).