You can use a Xpath constraint in your retrieve if it is the same object like [id = $Transient_Entity], however this won't work in the current setting because the Transactie and Transient_Entity objects are from different entities. So I suggest you make use of unique identifiers to identify your transactions, like a unique transaction number. Whenever you get a Transient_Entity object from your rest service, you can retrieve the first Transactie like you already did; you only need to add the UID to the Xpath retrieve.