hi all i tried to change readonly or disabled field value this what i do
1- i made Remaining amount Read Only and put onchange script on paid amount when change it will subtract the payment amount - paid amount and the result
put it in readonly field (Remaining amount ) but it dosnt work
I try hiddenField & DataField & Field but no result still the value not changed.
I try to disable the field but when i save its put zero in payment amount and remaining amount 
document.getElementById('Paym_RemAmount').value=Num;
document.getElementById('_HEDDRNPaym_RemAmount').value=Num;
document.getElementById('_DataPaym_RemAmount').value=Num;
Put nothing changed
