Issues with Custom Buttom in a Salesforce Opportunity

Hi Rafi,

You can execute the below logic on your custom button :

<apex:page standardController="Opportunity">
<apex:detail Subject="{!Opportunity.Id}" relatedList="true" title="true" oncomplete="Javascript:location.reload();" />

<script src = "//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
<script type = "text/Javascript">

var jquery = $.noConflict();
jquery(document).ready(function(){

var $buttons = jQuery("[name= 'discount_approval']");
if ('{!Opportunity.StageName}' != 'Proposal/Price Quote' && '{!Opportunity.StageName}' != 'Negotiation/Review'){

$buttons.toggleClass('btnDisabled', true).attr('disabled', 'disabled');
}
});
</script>
</apex:page>