Fixed balance list.

app/Models/Tag.php

@@ -92,6 +92,22 @@ class Tag extends Model
         throw new NotFoundHttpException;
     }
 
+    /**
+     * @param Tag $tag
+     *
+     * @return string
+     */
+    public static function tagSum(Tag $tag): string
+    {
+        $sum = '0';
+        /** @var TransactionJournal $journal */
+        foreach ($tag->transactionjournals as $journal) {
+            bcadd($sum, TransactionJournal::amount($journal));
+        }
+
+        return $sum;
+    }
+
     /**
      * @codeCoverageIgnore
      *