discount problem

it is really strange because when my colleague is accepting payment based on the invoices which had been made, the system automatically gives an discount. In the mean time, the discount taken field sometimes is empty and sometimes is not. Thus some deviations are caused. How should I solve this problem.

My solution is: using a form-level based client script, when the page is initialized, I could reset the field values to be zero. The form is based on Standard Customer Payment, there are tabs and sublists. Is my solution feasible? How could I obtain those information? For example, obtain the line items.