diff --git a/addons/purchase/models/purchase.py b/addons/purchase/models/purchase.py
index 21238bfe978..786d7c7b951 100644
--- a/addons/purchase/models/purchase.py
+++ b/addons/purchase/models/purchase.py
@@ -1202,7 +1202,7 @@ class PurchaseOrderLine(models.Model):
     @api.depends('product_qty', 'product_uom')
     def _compute_price_unit_and_date_planned_and_name(self):
         for line in self:
-            if not line.product_id:
+            if not line.product_id or line.invoice_lines:
                 continue
             params = {'order_id': line.order_id}
             seller = line.product_id._select_seller(
diff --git a/addons/purchase/tests/test_purchase_invoice.py b/addons/purchase/tests/test_purchase_invoice.py
index c28901d111a..e10c82da363 100644
--- a/addons/purchase/tests/test_purchase_invoice.py
+++ b/addons/purchase/tests/test_purchase_invoice.py
@@ -457,3 +457,57 @@ class TestPurchaseToInvoice(AccountTestInvoicingCommon):
         self.assertEqual(line.qty_invoiced, 10)
         line.qty_received = 15
         self.assertEqual(line.qty_invoiced, 10)
+
+    def test_on_change_quantity_price_unit(self):
+        """ When a user changes the quantity of a product in a purchase order it
+        should only update the unit price if PO line has no invoice line. """
+
+        supplierinfo_vals = {
+            'partner_id': self.partner_a.id,
+            'price': 10.0,
+            'min_qty': 1,
+            "product_id": self.product_order.id,
+            "product_tmpl_id": self.product_order.product_tmpl_id.id,
+        }
+
+        supplierinfo = self.env["product.supplierinfo"].create(supplierinfo_vals)
+        po_form = Form(self.env['purchase.order'])
+        po_form.partner_id = self.partner_a
+        with po_form.order_line.new() as po_line_form:
+            po_line_form.product_id = self.product_order
+            po_line_form.product_qty = 1
+        po = po_form.save()
+        po_line = po.order_line[0]
+
+        self.assertEqual(10.0, po_line.price_unit, "Unit price should be set to 10.0 for 1 quantity")
+
+        # Ensure price unit is updated when changing quantity on a un-confirmed PO
+        supplierinfo.write({'min_qty': 2, 'price': 20.0})
+        po_line.write({'product_qty': 2})
+        self.assertEqual(20.0, po_line.price_unit, "Unit price should be set to 20.0 for 2 quantity")
+
+        po.button_confirm()
+
+        # Ensure price unit is updated when changing quantity on a confirmed PO
+        supplierinfo.write({'min_qty': 3, 'price': 30.0})
+        po_line.write({'product_qty': 3})
+        self.assertEqual(30.0, po_line.price_unit, "Unit price should be set to 30.0 for 3 quantity")
+
+        po.action_create_invoice()
+
+        # Ensure price unit is NOT updated when changing quantity on PO confirmed and line linked to an invoice line
+        supplierinfo.write({'min_qty': 4, 'price': 40.0})
+        po_line.write({'product_qty': 4})
+        self.assertEqual(30.0, po_line.price_unit, "Unit price should be set to 30.0 for 3 quantity")
+
+        with po_form.order_line.new() as po_line_form:
+            po_line_form.product_id = self.product_order
+            po_line_form.product_qty = 1
+        po = po_form.save()
+        po_line = po.order_line[1]
+
+        self.assertEqual(235.0, po_line.price_unit, "Unit price should be reset to 235.0 since the supplier supplies minimum of 4 quantities")
+
+        # Ensure price unit is updated when changing quantity on PO confirmed and line NOT linked to an invoice line
+        po_line.write({'product_qty': 4})
+        self.assertEqual(40.0, po_line.price_unit, "Unit price should be set to 40.0 for 4 quantity")