diff --git a/cinder/volume/manager.py b/cinder/volume/manager.py
index eff70bec100..33cd396160a 100644
--- a/cinder/volume/manager.py
+++ b/cinder/volume/manager.py
@@ -763,11 +763,12 @@ class VolumeManager(manager.CleanableManager,
 
         # To backup a snapshot or a 'in-use' volume, create a temp volume
         # from the snapshot or in-use volume, and back it up.
-        # Get admin_metadata to detect temporary volume.
+        # Get admin_metadata (needs admin context) to detect temporary volume.
         is_temp_vol = False
-        if volume.admin_metadata.get('temporary', 'False') == 'True':
-            is_temp_vol = True
-            LOG.info("Trying to delete temp volume: %s", volume.id)
+        with volume.obj_as_admin():
+            if volume.admin_metadata.get('temporary', 'False') == 'True':
+                is_temp_vol = True
+                LOG.info("Trying to delete temp volume: %s", volume.id)
 
         # The status 'deleting' is not included, because it only applies to
         # the source volume to be deleted after a migration. No quota
diff --git a/releasenotes/notes/fix-quota-deleting-temporary-volume-274e371b425e92cc.yaml b/releasenotes/notes/fix-quota-deleting-temporary-volume-274e371b425e92cc.yaml
new file mode 100644
index 00000000000..33d31982278
--- /dev/null
+++ b/releasenotes/notes/fix-quota-deleting-temporary-volume-274e371b425e92cc.yaml
@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fix a quota usage error triggered by a non-admin user backing up an
+    in-use volume. The forced backup uses a temporary volume, and quota
+    usage was incorrectly updated when the temporary volume was deleted
+    after the backup operation completed.
+    Fixes `bug 1778774 <https://bugs.launchpad.net/tripleo/+bug/1778774>`__.