diff --git a/addons/purchase/models/purchase.py b/addons/purchase/models/purchase.py
index 762163be252..4bebaaabbaa 100644
--- a/addons/purchase/models/purchase.py
+++ b/addons/purchase/models/purchase.py
@@ -1207,6 +1207,12 @@ class PurchaseOrderLine(models.Model):
 
             # If not seller, use the standard price. It needs a proper currency conversion.
             if not seller:
+                unavailable_seller = line.product_id.seller_ids.filtered(
+                    lambda s: s.partner_id == line.order_id.partner_id)
+                if not unavailable_seller and line.price_unit and line.product_uom == line._origin.product_uom:
+                    # Avoid to modify the price unit if there is no price list for this partner and
+                    # the line has already one to avoid to override unit price set manually.
+                    continue
                 po_line_uom = line.product_uom or line.product_id.uom_po_id
                 price_unit = line.env['account.tax']._fix_tax_included_price_company(
                     line.product_id.uom_id._compute_price(line.product_id.standard_price, po_line_uom),
diff --git a/addons/purchase_requisition/tests/test_purchase_requisition.py b/addons/purchase_requisition/tests/test_purchase_requisition.py
index 03f23ecc25c..7ae68d4ade1 100644
--- a/addons/purchase_requisition/tests/test_purchase_requisition.py
+++ b/addons/purchase_requisition/tests/test_purchase_requisition.py
@@ -217,3 +217,35 @@ class TestPurchaseRequisition(TestPurchaseRequisitionCommon):
         groups = self.env['purchase.order.group'].search([('order_ids', 'in', pos.ids)])
         self.assertEqual(len(po_5.alternative_po_ids), 0, "Last PO should auto unlink from itself since group should have auto-deleted")
         self.assertEqual(len(groups), 0, "The group should have auto-deleted")
+
+    def test_09_alternative_po_line_price_unit(self):
+        """Checks PO line's `price_unit` is keep even if a line from an
+        alternative is chosen and thus the PO line's quantity was set to 0. """
+        # Creates a first Purchase Order.
+        po_form = Form(self.env['purchase.order'])
+        po_form.partner_id = self.res_partner_1
+        with po_form.order_line.new() as line:
+            line.product_id = self.product_09
+            line.product_qty = 1
+            line.price_unit = 16
+        po_1 = po_form.save()
+
+        # Creates an alternative PO.
+        action = po_1.action_create_alternative()
+        alt_po_wizard_form = Form(self.env['purchase.requisition.create.alternative'].with_context(**action['context']))
+        alt_po_wizard_form.partner_id = self.res_partner_1
+        alt_po_wizard_form.copy_products = True
+        alt_po_wizard = alt_po_wizard_form.save()
+        alt_po_wizard.action_create_alternative()
+
+        # Set a lower price on the alternative and choses this PO line.
+        po_2 = po_1.alternative_po_ids - po_1
+        po_2.order_line.price_unit = 12
+        po_2.order_line.action_choose()
+
+        self.assertEqual(
+            po_1.order_line.product_uom_qty, 0,
+            "Line's quantity from the original PO should be reset to 0")
+        self.assertEqual(
+            po_1.order_line.price_unit, 16,
+            "Line's unit price from the original PO shouldn't be changed")
diff --git a/addons/purchase_stock/models/purchase.py b/addons/purchase_stock/models/purchase.py
index cb32d841d68..82148aebca7 100644
--- a/addons/purchase_stock/models/purchase.py
+++ b/addons/purchase_stock/models/purchase.py
@@ -453,13 +453,13 @@ class PurchaseOrderLine(models.Model):
             price_unit = self.taxes_id.with_context(round=False).compute_all(
                 price_unit, currency=self.order_id.currency_id, quantity=qty, product=self.product_id, partner=self.order_id.partner_id
             )['total_void']
-            price_unit = float_round(price_unit / qty, precision_digits=price_unit_prec)
+            price_unit = price_unit / qty
         if self.product_uom.id != self.product_id.uom_id.id:
             price_unit *= self.product_uom.factor / self.product_id.uom_id.factor
         if order.currency_id != order.company_id.currency_id:
             price_unit = order.currency_id._convert(
                 price_unit, order.company_id.currency_id, self.company_id, self.date_order or fields.Date.today(), round=False)
-        return price_unit
+        return float_round(price_unit, precision_digits=price_unit_prec)
 
     def _prepare_stock_moves(self, picking):
         """ Prepare the stock moves data for one order line. This function returns a list of