When an purchase invoice or purchase credit memo is created by a webservice the document date on the document in BC is based on the actual document date mentioned on the physical invoice document.

The posting date at first is equal to the system date at the moment of the document creation.

The document date in this situation is a given. It is the date on the actual invoice. The posting date could still change. For example because after the document approval the posting date is not within the allowed posting period. In this case you have to change the posting date but the document date should remain the same.

As it is now the document date changes when you change the posting date. The user has to always change the document date back to the original value.

It would be more use friendly to make the two date fields on the purchase invoice and purchase credit memo independent of each other.