A senior Mufti says that the Dua, „Allahumma antas salaam…‟ which we make after Fardh Salaat is based on a dhaeef Hadith.

Answer

So what? The senior Mufti is off the mark. Even senior Muftis sometimes speak gibberish. Even if the Hadith is 'dhaeef', the Dua is the standard practice of all the Akaabir Ulama who were not bid'atis. So, we set aside what the senior Mufti said.