As a synonym of bēlu “lord”, the present form must go back to WS *baˁl -. Following SAD 1 7, one has to assume that a nisbah suffix was attached to *baˁl -, even if this is rather unusual.
In the phrase di-báḥľe, the basesbaˁ- and báḥľe are in complementary distribution: the former is used before pronominal suffixes, the latter before nouns.