diff --git a/addons/stock/models/stock_move.py b/addons/stock/models/stock_move.py
index 2aa5f4326ee..393b0a31c28 100644
--- a/addons/stock/models/stock_move.py
+++ b/addons/stock/models/stock_move.py
@@ -918,11 +918,15 @@ class StockMove(models.Model):
                 if not move.move_orig_ids:
                     if move.procure_method == 'make_to_order':
                         continue
+                    # If we don't need any quantity, consider the move assigned.
+                    need = move.product_qty - move.reserved_availability
+                    if float_is_zero(need, precision_rounding=move.product_id.uom_id.rounding):
+                        assigned_moves |= move
+                        continue
                     # Reserve new quants and create move lines accordingly.
                     available_quantity = self.env['stock.quant']._get_available_quantity(move.product_id, move.location_id)
                     if available_quantity <= 0:
                         continue
-                    need = move.product_qty - move.reserved_availability
                     taken_quantity = move._update_reserved_quantity(need, available_quantity, move.location_id, strict=False)
                     if float_is_zero(taken_quantity, precision_rounding=move.product_id.uom_id.rounding):
                         continue
diff --git a/addons/stock/tests/test_move.py b/addons/stock/tests/test_move.py
index df57065b8d7..8326328bc95 100644
--- a/addons/stock/tests/test_move.py
+++ b/addons/stock/tests/test_move.py
@@ -925,6 +925,31 @@ class StockMove(TransactionCase):
         self.assertEqual(self.env['stock.quant']._get_available_quantity(self.product2, self.customer_location), 12.0)
         self.assertEqual(len(self.env['stock.quant']._gather(self.product2, self.customer_location)), 12)
 
+    def test_availability_8(self):
+        """ Test the assignment mechanism when the product quantity is decreased on a partially
+            reserved stock move.
+        """
+        # make some stock
+        self.env['stock.quant']._update_available_quantity(self.product1, self.stock_location, 3.0)
+        self.assertAlmostEqual(self.product1.qty_available, 3.0)
+
+        move_partial = self.env['stock.move'].create({
+            'name': 'test_partial',
+            'location_id': self.stock_location.id,
+            'location_dest_id': self.customer_location.id,
+            'product_id': self.product1.id,
+            'product_uom': self.uom_unit.id,
+            'product_uom_qty': 5.0,
+        })
+
+        move_partial._action_confirm()
+        move_partial._action_assign()
+        self.assertAlmostEqual(self.product1.virtual_available, -2.0)
+        self.assertEqual(move_partial.state, 'partially_available')
+        move_partial.product_uom_qty = 3.0
+        move_partial._action_assign()
+        self.assertEqual(move_partial.state, 'assigned')
+
     def test_unreserve_1(self):
         """ Check that unreserving a stock move sets the products reserved as available and
         set the state back to confirmed.
@@ -3767,4 +3792,3 @@ class StockMove(TransactionCase):
         picking.button_validate()
         self.assertEqual(self.env['stock.quant']._get_available_quantity(self.product1, self.stock_location), 0)
         self.assertEqual(self.env['stock.quant']._get_available_quantity(self.product1, self.customer_location), 2)
-