Karl Lindner is the welcoming committee representative for the Claybourne Park association in A Raisin in the Sun. He is the only white person that has a speaking role in the play, and he is a stand-in for the quiet, polite racism of middle-class Chicago. Karl plays an essential role in the play because he is acting as a test for the Younger family, specifically for Walter Lee.
Lindner’s name is possibly symbolic for a few reasons. First is that Karl could be a reference to Hansberry's father, who was a real estate broker and businessman—but that doesn’t seem to fit with his character. Instead, I think Karl is a play off the pejorative term “Mister Charlie” because Karl is a nickname for Charles. This explanation for his name makes more sense because Karl Lindner is offering them a deal that is telling them they are lesser, and “Mr. Charlie” was the name used to refer to a slave owner.
Hansberry lets on to the link between the name Karl and “the man” when Walter Lee calls him “Mistuh Charlie” in act three,
WALTER: (Breathing hard) Made a call.
MAMA: To who, son?
WALTER: To The Man. (He heads for his room)
MAMA: What man, baby?
WALTER: (Stops in the door) The Man, Mama. Don’t you know who The Man is?
RUTH: Walter Lee?
WALTER: The Man. Like the guys in the streets say—The Man. Captain Boss—Mistuh Charley . . . Old Cap’n Please Mr. Bossman . . .
BENEATHA: (Suddenly) Lindner!
WALTER: That’s right! That’s good. I told him to come right over.
Walter refers to Lindner as “the man”—a term for the idea that white people run the world. Lindner is a stand-in for white society, with its money and its ability to dictate how things are run. He also clues us into what the name Karl represents when he refers to him as “Mistuh Charley”—a clear connection to the use of the term during slavery. The Youngers live in Chicago, not in the south, and the use of that inflection is a clear indication that he is connecting Lindner not just to white power, but to slavery—as "Mister Charlie" was a nickname slaves used for their masters.
Mama makes the same connection when she tells Walter that making a deal with Lindner would set their family back further than slavery because it means they are accepting that they are less than the white people. She says,
MAMA: Son—I come from five generations of people who was slaves and sharecroppers—but ain’t nobody in my family never let nobody pay ’em no money that was a way of telling us we wasn’t fit to walk the earth. We ain’t never been that poor. (Raising her eyes and looking at him) We ain’t never been that—dead inside.
The name Karl is a clear indication that despite his outward demeanor, his Faustian deal would cost them much more than their house: it would cost them their dignity and their souls. To take his deal, to accept the money, would mean that they are acknowledging their inherent worthlessness—something they didn't even have to do when they were in slavery. The clear connection to slavery and the deal Lindner offers is an indication that his name is related to the term "Mister Charlie". That seems to be the primary significance of the name Karl Lindner.