Sum and join 2 tables

Question

I want to sum and join 2 tables. I am getting a wrong answer. What am i doing wrong please

$stmt = $pdo->query("
   SELECT 
   a.prod_name, a.prod_size,
   b.prod_name, b.prod_size,
   SUM(b.qty_bought) qty_bot,
   SUM(a.prod_qty) qty_received
   FROM tbl_distribution a
   JOIN tbl_sales_bar b
   ON a.staff_id = b.staff_id
   WHERE a.staff_id = '$_GET[id]'
   GROUP BY  b.prod_id
");
WHILE($row = $stmt->fetch(PDO::FETCH_ASSOC)){
  echo '<tr>
           <td>'.$row["prod_name"].'</td>           
           <td>'.$row["prod_size"].'</td>
           <td>'.$row["qty_received"].'</td>
           <td>'.$row["qty_bot"].'</td>
           <td></td>
           </tr>';
}

what i want is to get the sum of quantity bought and the sum of product quantity so that when i minus the product quantity for the quantity bought, i will get the balance which is the difference between the product quantity - quantity bought = available quantity

Thanks

Edit: let me use images to show what i intend to have as result:

thanks