I work on a contract that has the following payment terms:
1. 30% as a down payment  at contract signing (to prepare the Execution drawing, installing the drainage System)

2. 25% before requesting the Indoor unit and installing the air Duct

3. 25% before requesting the outdoor unit and installing the copper pipes

4. 10% before installing the thermostat and diffusers 

5. 10% before starting up and commissioning 

There is no date to add due to the scope of work, and the customer requests the next phase after finishing.

-How can I issue a VAT invoice for the down payment if I d payment if I did not do any work as per the customer's request?

-How to add the above payment terms in the Quotation and keep the system understanding the phase with percentage?