Can a professor take a full-time 12 months industry job (get fully paid) while on one-year sabbatical? Assuming the home university only pays 1/2 of 9-months salary in the period of one-year sabbatical.

Can it be legally allowed? or does it violate any rules in general? What would be the worst consequence if doing so?  

Thanks.
The only legal rules that are likely to apply here are the contractual rules for the terms of the professor's employment with the university, combined with the university's policy/contract relating to sabbaticals.  Whether or not this is allowed would depend on the terms of the sabbatical, as set out in the appropriate university policies and employment contract.