Is it permissible to interrupt the prayer after remembering to perform it in combination (jama‘an), and what is the expiation for that if it is not permissible?

If a traveler combines Zuhr with Asr as a jama‘ taqdim (early combination) and then reaches his city at the time of Asr or before it, he is not obligated to repeat the prayer, because his responsibility for it has been discharged. Some scholars, however, hold the view that it is preferable to repeat Asr as long as its time remains. According to the Hanbalis, it is better not to perform the Asr prayer (again) because it would be considered a forbidden supererogatory (nafl) prayer during times when prayer is prohibited.