It seems that the Prophet (sws) has allowed giving of Sadaqah on someone’s behalf in the following Hadīth? Isn’t this a form of Iīthāl-i-Thawāb you have criticized in one of your answers?
The mother of Sa‘ad Ibn ‘Ubādah died in his absence. He said: O Allah's Apostle! My mother died in my absence; will it be of any benefit for her if I give Sadaqah on her behalf? The Prophet said: Yes. Sa‘ad said: I make you a witness that I gave my garden called al-Makhraf in charity on her behalf. (Musnad Ahmad Ibn Hambal, ‘an Musnad Abī Hishām)
If all the texts of the above Hadīth are collected and analyzed, it becomes evident that the mother of Sa`ad Ibn ‘Ubādah (rta) had vowed to give some money in charity and before she could, she died. It is this vowed (nadhr) Sadaqah which her son had asked the Prophet (sws) about and to which the Prophet (sws) had given his consent.
This is similar to paying off a debt which is outstanding in a dead person’s name, and in no way can be regarded as Iīthāl-i-Thawāb.