English to Italian Other Law: Contract(s) private agreement between buyer and supplier

Initiatives on changing the delivery item may be made by both parties, whereby only xxxx may demand a change. As a general rule they are subject to written approval by xxxx and detailed presentation of the effects of the change at the supplier's cost and on the supplier's dates. Deliveries on the basis of the changed delivery item may only be carried out if the supplier is in possession of the written release or variation authorization from xxxx.

In this context the "written release" should be an authorization notice, but I don't know whether an italian specific term exists