It will be a standard feature in v18. 

The invoicing wizard will be improved, also adding the possibility of creating down payments for purchase orders, more info: https://x.com/thefuturelens/status/1811638004538110358

Hi,

Check the related forum

https://www.odoo.com/sl_SI/forum/pomoc-1/how-to-enter-a-prepayment-for-a-vendor-103753

Regards

You register an invoice to a Prepayment account.  When you get a final invoice, you reconcile it with the prepayment invoice.

It is a disadvantage in Odoo that you cannot link a prepayment invoice to the purchase order, which means you must remember  - make a not or a reminder on the purchase order or the journal entry for follow up.