In The Contender, why does Mr. Epstein tell Alfred he is fired until tomorrow?

This is simply Mr. Epstein's playful way of telling Alfred that he can have the rest of the day off work.
Alfred's still pretty beat after his second bout with Griffin the night before, so he isn't in the right physical condition for work. Not only that, but his mind's still back in the ring; he can't stop thinking about the crunching sound of his fist hitting against Griffin's jaw.
It's no wonder, then, that Alfred starts making some pretty basic mistakes while working on the cash register. Twice, he rings up the wrong totals and Jake has to come and unlock the cash register and correct the receipt roll. Mr. Epstein recognizes that Alfred's better off getting some rest, so he lets him go until 8:30 the following morning, when he'll need to turn up bright and early for work again. Hopefully then he'll be in the right frame of mind.