diff --git a/addons/sale/models/account_move.py b/addons/sale/models/account_move.py
index ce4a1d411ef..0e20b8f4c82 100644
--- a/addons/sale/models/account_move.py
+++ b/addons/sale/models/account_move.py
@@ -6,6 +6,25 @@ from odoo.exceptions import UserError
 from odoo.tools import float_compare, float_is_zero
 
 
+class AccountMove(models.Model):
+    _inherit = 'account.move'
+
+    def action_post(self):
+        #inherit of the function from account.move to validate a new tax and the priceunit of a downpayment
+        res = super(AccountMove, self).action_post()
+        line_ids = self.mapped('line_ids').filtered(lambda line: line.sale_line_ids.is_downpayment)
+        for line in line_ids:
+            try:
+                line.sale_line_ids.tax_id = line.tax_ids
+                #To keep positive amount on the sale order and to have the right price for the invoice
+                #We need the - before our untaxed_amount_to_invoice
+                line.sale_line_ids.price_unit = -line.sale_line_ids.untaxed_amount_to_invoice
+            except UserError:
+                # a UserError here means the SO was locked, which prevents changing the taxes
+                # just ignore the error - this is a nice to have feature and should not be blocking
+                pass
+        return res
+
 class AccountMoveLine(models.Model):
     _inherit = 'account.move.line'
 
diff --git a/addons/sale/models/sale.py b/addons/sale/models/sale.py
index a5b2988202c..d47d9f8f732 100644
--- a/addons/sale/models/sale.py
+++ b/addons/sale/models/sale.py
@@ -983,6 +983,8 @@ class SaleOrderLine(models.Model):
         for line in self:
             if line.state not in ('sale', 'done'):
                 line.invoice_status = 'no'
+            elif line.is_downpayment and line.untaxed_amount_to_invoice == 0:
+                line.invoice_status = 'invoiced'
             elif not float_is_zero(line.qty_to_invoice, precision_digits=precision):
                 line.invoice_status = 'to invoice'
             elif line.state == 'sale' and line.product_id.invoice_policy == 'order' and\
@@ -1030,7 +1032,7 @@ class SaleOrderLine(models.Model):
             else:
                 line.qty_to_invoice = 0
 
-    @api.depends('invoice_lines.move_id.state', 'invoice_lines.quantity')
+    @api.depends('invoice_lines.move_id.state', 'invoice_lines.quantity', 'untaxed_amount_to_invoice')
     def _get_invoice_qty(self):
         """
         Compute the quantity invoiced. If case of a refund, the quantity invoiced is decreased. Note
@@ -1044,8 +1046,9 @@ class SaleOrderLine(models.Model):
                 if invoice_line.move_id.state != 'cancel':
                     if invoice_line.move_id.type == 'out_invoice':
                         qty_invoiced += invoice_line.product_uom_id._compute_quantity(invoice_line.quantity, line.product_uom)
-                    elif invoice_line.move_id.type == 'out_refund':
-                        qty_invoiced -= invoice_line.product_uom_id._compute_quantity(invoice_line.quantity, line.product_uom)
+                    elif invoice_line.move_id.type == 'out_refund' :
+                        if not line.is_downpayment or line.untaxed_amount_to_invoice == 0 :
+                            qty_invoiced -= invoice_line.product_uom_id._compute_quantity(invoice_line.quantity, line.product_uom)
             line.qty_invoiced = qty_invoiced
 
     @api.depends('price_unit', 'discount')