Is it permissible for the one being led in prayer (ma'mum) to differ from the imam in the intention of combining prayers (jam'), such that the imam combines while the ma'mum withdraws after the first prayer, whether the imam has an excuse for combining or not?

Combining two prayers is not obligatory except in Arafat and Muzdalifah; it is permissible if there is an excuse. There is no harm for the ma'mum (follower) in not combining prayers with the imam (leader) if the ma'mum has no excuse, and his first prayer is valid. It is not permissible to combine prayers with the imam unless both the imam and the ma'mum have an excuse that permits combining. If the imam combines prayers without an excuse, his second prayer is invalid because it was performed before its prescribed time.