according to the Rambam a”z 7:9 a”z asur bemashehu so
how is there bitul?
You mean the Rambam of Takruves AZ (Halacha 16). See the Bais Shlomo (link below) in the beg. of Teshuvah 30 addressing that Rambam.
http://hebrewbooks.org/pdfpager.aspx?req=611&st=&pgnum=41&hilite=