Should a 2.5% or 5% be offered as a rebate at the end of the year.

AssalamuAlaykum. Need some advice. Is it permissible to offer customers a 2.5% or 5% rebate at the end of the year. We have schools that buy from us and they ask for donations all the time. So we looking at maybe offering a flat rebate of 5% to the schools on the amount that they buy during the course of the year. Your hasty response will be appreciated.
Jazakallah