store negative amount in Transaction when buying something

cash/models.py

@@ -45,7 +45,7 @@ class Account(models.Model):
 				descr += '%dx %s\n' % (amount, product.name)
 
 			transaction = Transaction(account=self, subject=BUY_SUBJECT,
-			                          amount=total_value, description=descr)
+			                          amount=(-total_value), description=descr)
 			transaction.save()
 
 			return True