Thank you all for your inputs. Problem's solved the aikido way, everybody on win-win situation.
A. Old sensei did not allow sempai to leave, sempai will be graded and tested for BB -
I'm glad the problem worked out, but if I were the student, I would be asking myself and my Sensei exactly what it is he wants me to learn / master in order to pass the test. Grades are not awarded for time spent training, for help around the dojo or administrative tasks. Grades are awarded for ability. So find out what abilities are needed.
If I was awarded a grade for anything but ability, I would feel insulted. I trust my Sensei to judge my improvement because he is vastly more experiences than me.
On the other hand, if he makes requests of me for administrative assistance, then I choose to say 'yes' or 'no' and expect that either choice will have no effects on my training or grading.