Tax Invoice against Sales Order but not itemized 1 line called payment

Hello everyone,

I am trying to figure out a workaround for the vat tax point.

The client will pay a deposit of 80%, but i want to invoice for that 80% with one item called payment. So not a stock Item.

I want to do this so i am not liable for the outstanding tax on the whole amount.

So if the Sales Order is 100 000

They pay me 80 000

Outstanding is 20 000

On the tax invoice i don’t want it to be detailed with items that i have to delete to make it 80 000.
Can i put an item called payment, send them that invoice and will the accounting balance correctly?

I only want to pay the vat on the 80 000 first, then 3-4 months when the project is completed pay the vat on the 20 000.

Does the stock balance against the delivery note or does the stock have to balance on the Invoice?

Please help as this is fairly urgent