Script to populate invoice items

I write function which populate items to invoice on custom form 109.
When I adding new line item I have a problem.
I set all fields but field amount has bean cleaned by some script.
I suppose some ajax script triggers on item change and clean amount field.
What can I do with this?
My function.

function populateRecords() {
    var entity = nlapiGetFieldValue('entity');
    if (entity) {
        var filters = new Array();
        filters[0] = new nlobjSearchFilter( 'custrecord_cm_source', null, 'anyof', nlapiGetFieldValue('entity'));

        var columns = new Array();
        columns[0] = new nlobjSearchColumn( 'internalid' );

        var searchresults = nlapiSearchRecord( 'customrecordcomm_rec', 'customsearch_current_payment', filters, columns);
        if (searchresults != null) {
            for (var h = 0; h<searchresults.length;h++) {