Current behavior:
When you refund a downpayment in the PoS, the downpayment is not added
to the original order.

Steps to reproduce:
- Create an order in the sales app
- Open the PoS and make a downpayment for the order
- Refund the downpayment you just made
- Go back to the original order in the sales app, the downpayment is
  correctly added, but the refund of the downpayment is not added.
- The order should contain : The original product, the downpayment and
  the refund of the downpayment.

opw-3275708

closes odoo/odoo#121160

Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>