Hello,
I have gone through your question and did same operation in my environment and find out some facts about Document Date.
Document Date is the date of the document you are creating, that is why it's taking current date. but if you want to change the Document from Web Service, then you have to use Microsoft Web API.
As you can see I have used the same web service which you are using, so Business Central by default adding Current Date in Document Date field.

Now, I have used Web API of Microsoft to Create Invoice, In which they have provided one property called Invoice Date, It will change both Document and Posting Date.
Below is the example.
Postman Call:

Response:



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Thanks & Regards,
Ankit Shah(MCT-Microsoft Certified Trainer || Dynamics 365/Business Development Manager)
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