No, on those facts it's not illegal. The long-shot argument would be based upon unlawful discrimination -- if the benefit is being given to one class of workers based upon race, sex, color, religion, or another protected status, and being denied to others who were a different race, sex, color, religion, etc.

It may simply be a matter of trust. The employer may have more confidence in the worker who received the paycheck a day early that he won't deposit the check early, than he does in other workers. It may be a one-time deal that the employer doesn't intend to repeat, even for that single employee. But no matter what motivated the employer, outside of the aforementioned civil rights contexts, it's not unlawful.