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Seeing as he is a Navi Sheker, how come that the sign that he gave came true?
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Seforno and Rashbam: Because he employed witchcraft or something similar (a Ru'ach Tum'ah - Terafim [household idols] or the sorcery of Ov or Yid'oni, and Hashem gave them the power to know the future in order to test Yisrael, to test Yisrael - Rashbam). 1
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Rashbam: As to whether hey will fulfill the Mitzvah of "Lo Yih'yeh b'cha Me'onein, u'Menachesh u'Mechashef ? Tamim Tih'yeh im Hashem Elokecha" - Parshas Shoftim, 18:10-13.
