Ifham al Yahud

I recently read a twelfth century book, titled Ifham al Yahud (silencing the Jews), written by a Jewish mathematician from Morocco named Al-Samawal al-Maghribi. This man unfortunately publicly converted to Islam subsequent to the death of his father and wrote Ifham al Yahud as a polemic against Judaism and the veracity of the torah. Most of his arguments are very weak, but he made three points that I have not been able to respond to and am trying to find an explanation for. https://archive.org/details/28921238SilencingTheJews/page/n35/mode/2up. On the forty-first page he argues that the torah has an internal contradiction chas v’sholom because of the fact that G d chose the levi’im for service in numbers 8-18 as opposed to the bechorim, which according to him contradicts the issur of לא תוסיפו ולא תגרעו because the bechorim were initially intended for the role that Hashem gave to the levi’im. How can this be refuted?