Is it incumbent upon one who fasted for two days and whose deed was then rendered void on the third day due to his abandoning the 'Asr prayer, to re-fast that third day, or is repentance sufficient?

The "nullification" mentioned concerning the Asr prayer is a matter of scholarly disagreement. Ibn Hajar, may Allah have mercy on him, favored interpreting it as a deterrent. If a Muslim repents with sincere repentance, that is sufficient for him, and his deeds will not be nullified. He is also not required to make up the day he missed the prayer.