Holding that "[i]f the discriminatee accepts significantly lower-paying work too soon after the discrimination in question, he may be subject to a reduction in back pay on the ground that he willfully incurred a loss by accepting an `unsuitably' low paying position"
Noting that the period may be shortened by a showing that the salt accepted a promotion from the Union during the back pay period which removed him from the field
Holding that an "employer is not liable for back pay during the periods that an improperly discharged employee is unavailable to work due to a disability"