Operation processes on invoices

app/ValueObjects/Amount.php

@@ -40,4 +40,8 @@ class Amount {
     public function addAmount(Amount $amount) : void {
         $this->amount += $amount->getAmount();
     }
+
+    public function subtractAmount(Amount $amount) : void {
+        $this->amount -= $amount->getAmount();
+    }
 }