What is the ruling regarding substitution for the Imam if he breaks his ritual purity (Hadath) after delivering the sermon?

Primary text

A narration from Ahmad states that if the Imam breaks ritual purity after delivering the sermon and appoints someone to lead the rest of the prayer, the congregation's prayer is incomplete (only four rak'ahs), unless the appointed person repeats the sermon before leading the remaining two rak'ahs. This ruling is established because this specific scenario (substitution without immediate re-sermon) was not reported from the Prophet or any of his successors, and this represents the primary established Madhhab (ruling).