Is it permissible for a person who has given charity to pray one rak'ah with the intention of both the regular Sunnah prayer and giving charity, if he joins someone who is late for the 'Isha' prayer in the last rak'ah before rukoo'?

For a person performing a supererogatory prayer to follow someone performing an obligatory prayer is permissible according to the majority of scholars, because the Imam's intention includes what is in the follower's intention, and more. This is supported by the Prophet's (peace and blessings be upon him) saying: "Is there no man who would give charity to this one and pray with him?" Therefore, there is no harm for a worshiper to intend two rak'ahs of a supererogatory prayer behind someone praying 'Isha. If the Imam has one rak'ah remaining, he should join him and, after the Imam says the taslim, pray one more rak'ah. By doing so, he will receive the reward for his supererogatory prayer and the reward for giving charity to this person who was praying alone.