(1) The obligor is obliged to perform to the new obligee only against delivery of a document concerning the assignment issued by the previous obligee. Notice of termination or a dunning letter from the new obligee is ineffective if it occurs without presentation of such a document and if the obligor rejects it without undue delay for that reason.
(2) These provisions are not applicable if the previous obligee notified the obligor of the assignment in writing.