"on rase gratis" is an old expression used in a peculiar context; when you're talking about a promise someone makes and surely won't be able to keep. If you were to translate it literally (which doesn't really make sense), it would be: "tomorrow we shave for free". Try to google it you'll find tons of examples.
I've heard it came from an old advertizing campaign. Barbers put signs saying "Tomorrow, free shavings!", people saw them and took note to come back the next day for their free shaving.
And on the morrow, after getting their shave, they were of course asked to pay. "But," people would complain, "wasn't it free today?" And they were replied, "check the sign, it says tomorrow, not today!"
True or not, the story explains the saying. That idiom is used sarcastically to imply a promise will never be kept.