I think your missing my point. Both words (if you think they are different) originate from the same root. I don't think blacks as a whole, collectively got together and decided that it was cool to replace "er" with "a". And now that word is acceptable to say amongst themselves.
First take a look at this for a little background:
http://dictionary.reference.com/search?q=ebonics
http://dictionary.reference.com/search?q=Black%20English
http://en.wikipedia.org/wiki/Ebonics
Lets go through a little ebonics:
"da" = "the"
"walkin'" = "walking"
"dat" = "that"
"wit" = "with"
Now here's some more that will show you my point:
"mutha" = "mothER"
"fucka" = "fuckER"
"brotha" = "brothER"
"gangsta" = "gangstER"
"youngsta" = "youngstER"
"sucka" = "SuckER"
Alright class, your turn:
"Nigga" = "Nigg__"
I wonder if I went up to my bio professor and called him an "Azzhole" if he'd be pissed?
By no stretch of the imagination am I racist, but it is rediculous that they use that word in reference to themselves. Notice you don't hear Jews calling each other "Crispies" or "gas-guzzlers". (For those jews out there, I mean absolutely no offense by that. I apologize, but only using it to make a point.)