trigger updateAmount on Invoice__c (after update)
{
  Set<Id> bookings = new Set<Id>();  
  List<Invoice__c> invoiceToUpdate = new List<Invoice__c>();
            
    for(Invoice__c inv:Trigger.new)
    {    
      if (inv.Booking__c != null) 
      {
        bookings.add(inv.Booking__c);
        system.debug('----------***Bookings****-------:'+bookings);
      }  
    }
   
   Invoice__c invoiceCancel = [SELECT Id,Status__c,Grand_Total__c FROM Invoice__c WHERE Booking__c IN:bookings
                      AND Status__c='Cancelled' ORDER BY createdDate DESC LIMIT 1];
                      
      System.debug('---------------invoiceCancel -------------'+invoiceCancel );  
                      
      //Decimal grandAmt1 =    
      //System.debug('---------------grandAmt1-------------'+grandAmt1);            
                      
   Invoice__c invoiceNew = [SELECT Id,Status__c,Grand_Total__c,Amount_Difference__c FROM Invoice__c WHERE Booking__c IN:bookings AND Status__c='New']; 
   
       //Decimal grandAmt2 =   
       //System.debug('---------------grandAmt2 -------------'+grandAmt2 );                   
   
       for(Invoice__c inv:trigger.new)
       {
          Decimal GrtTotal = invoiceNew.Grand_Total__c - invoiceCancel.Grand_Total__c;
          inv.Amount_Difference__c =  GrtTotal;
          System.debug('---------------Amount_Difference__c-------------'+inv.Amount_Difference__c );
          Invoice__c invv = new Invoice__c();
          invoiceNew.Amount_Difference__c = inv.Amount_Difference__c;
       } 
        update invoiceNew;
        update invoiceToUpdate;                  
    
 }