Hi Carlos,
To me its what your requirement is all about :
If any option is fine to be proposed then I would suggest to go with Proforma Invoice for full value and then run you actual billing with full value.
In order to handle the financial postings: Proforma - No Postings.
Actual Invoice - Postings will happen.
For the benefit of the customer have a Z- Statistical Condition type and have it during actual invoicing and try to print it and show ZERo amount to be paid.
The next possible scenario with Milestone Billing taken in to consideration:
Have two different materials -
MATERIAL 1: One - A Dummy material (No Stock) - Item Category with Billing Relevance as "I" and the item category will not have delivery possible. - Perform your Milestone Billing with this material.
MATERIAL 2: Physical product (With Stock) - Item Category with Billing Relevance as "A" and the item category will have delivery possible. - Actual Delivery of your material
Please let me know any further clarification.
Regards