5 answers

If you have your custom button setup to execute JavaScript you can try this: if(!{!ISPICKVAL(Invoice__c.Status__c, "Approved by Business Manager")}){
alert('Value can only be Approved by Business Manager');
}else{
window.open("/apex/VF_Example?sessionId"+{!$Api.Session_ID}+"&eid="+{!Invoice__c.Id}+"&ddpids=a0Z50000009UjxA&deploy=a0Y500000091MwT
&contactid=0035000002ZrYHU", "_blank");
}