Is it permissible for the intention of the Imam to differ from that of the worshiper (Ma'mum) in prayer?

The difference in intention between the Imam and the congregant does not matter. For example, if the Imam is praying a supererogatory prayer and the congregant an obligatory one, or vice-versa. The proof for this is that Mu'adh ibn Jabal used to pray the 'Isha prayer with the Messenger of Allah, peace and blessings be upon him, and then return to his people and lead them in that same prayer. So, it would be a supererogatory prayer for Mu'adh and an obligatory prayer for his people. It is permissible to follow in prayer in any two prayers whose actions do not differ. However, if their actions differ—such as an obligatory prayer with a solar eclipse prayer or a funeral prayer—then the congregant's prayer is invalid.