If(invcIds.size()>0){

    List<AggregateResult> ar = [Select invoiceit_s__Invoice__c invId, MAX(invoiceit_s__Payment_Date__c) pymntdt From invoiceit_s__Payment_Plan_Installment__c  Where invoiceit_s__Invoice__c IN:invcIds Group By invoiceit_s__Invoice__c ];
    for (AggregateResult a : ar){
     invcIddate.put((Id)a.get('invId'),(Date)a.get('pymntdt'));
    }
    system.debug('invcIddate'+invcIddate);
     for(invoiceit_s__Invoice__c inv : trigger.new) {

        if(invcIddate.size()>0 && invcIddate.containskey(inv.id)){
            inv.Final_Payment_Date__c = invcIddate.get(inv.id);
        }

     }

}