alert in javascript
 var n = parseInt(apex.item('P160_APPROVE_AMT').getValue().replace(/,/g, ''));
 var y = parseInt(apex.item('P160_REMAINING_AMOUNT').getValue().replace(/,/g, ''));

if (n > y) {
//alert(n);
$('#P160_APPROVE_AMT_error_placeholder').html("<b style='color: red'> Approve Amount Greter then Remaining Amount</b>!"),
$('#P160_APPROVE_AMT').focus('');
}
else {
   $('#P160_APPROVE_AMT_error_placeholder').html(""); 
}

replace ,
 select to_number(REGEXP_REPLACE('100,205.551', ',[^ ]', ''),'9999999999D999') from dual;