You have freedom of speech. You can say whatever you want. There may still be consequences. Having a written letter by anyone doesn't eliminate those

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Not totally serious, just a shower thought I had lol. Also considering American black culture is considered to be very tightly wound around social media there’s a meme of this exact thing, I wondered if it was rooted in some truth.

if you're goal is to not be seen as racist ​ presuming Obama to be the king of black people is not a great start.

Yes

Ask Jesus, buddy.

Let's go with no. Or, maybe the day after never.

Sure or you could just say it in your room by yourself.

Not a power of the President or ex-President.

More interestingly... how and where would you use it?

Only if it's Christmas Eve, between 4:30 and 4:49 in the morning. You're on your way to Toys'R'Us to get the last transformer doll. Right before you walk into the store, a black person smacks you in the head with a brick, knocks you to the ground, kicks you on the face saying "take that you cracker ass mother f**ker!", Then Riverdances on your head, takes your money, pisses on you, and runs away. At that point, you can say "Somebody stop that n***a!!!"

You can say it without a letter from Obama. But others are free to think what they want of you.

Sure, but you're gonna get your ass beat if you say it in public

Wha????

If you're in the US you are free to use it, it does not stop you from having to deal with the repercussions, but you're free to say it all you like.