Write up a quick memo to yourself documenting your manager’s use of an ethnic slur towards you. Get as many pertinent details as possible (like the time, date, as close to the exact phrase as possible). Memories fade fast, so get it in writing soon.
Then, file for unemployment. You can make a case for a constructive dismissal. This basically means that you quit because the manager created a hostile work environment by calling you an ethnic slur.
Here are some possible outcomes:
They don’t contest it, you get paid, and it costs them money.
They contest it, you follow through the entire appeals process, and it costs them time.
Honestly, getting unemployment insurance benefits in this situation is a real crap shoot. The point is to just make them deal with fighting the claim or paying the claim.
Good luck!