added amount due to email templates

app/Models/Document/Document.php

@@ -298,6 +298,18 @@ class Document extends Model
         return round($paid, $precision);
     }
 
+    /**
+     * Get the not paid amount.
+     *
+     * @return string
+     */
+    public function getAmountDueAttribute()
+    {
+        $precision = config('money.' . $this->currency_code . '.precision');
+
+        return round($this->amount - $this->paid, $precision);
+    }
+
     /**
      * Get the status label.
      *