I paid for a few months of a commuter benefit I never used, then left that employer. According to §1.132–9 in the CFR, these funds can't be returned by employers (see Q-14/A-14).
So I basically gave this money to my former employer, though I might have technically agreed to a salary reduction rather than allocating pre-tax money to the benefit.
Is there a tax deduction for losing funds this way? I assume it'd be similar to unused FSA funds at the end of the year.