SELECT h.e_id,
    h.cus,
    h.gd1-ifnull(t.gd1,0) as dif1,
    h.gd2-ifnull(t.gd2,0) as dif2
    FROM tabla1 h
      left join tabla2 t on h.e_id = t.e_id and h.cus = t.cus;
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This works