A major external conflict in Act III of A Raisin in the Sun is between Walter and his sister, Beneatha. As the scene opens, Beneatha is feeling lost and depressed about the loss of the money to pay for her schooling, and she is feeling very uncertain about her future as a result. Though Asagai tries to challenge her to keep dreaming and to keep moving forward - and to even consider coming to Africa with him to continue pursuing her dreams - she holds on to her anger toward Walter, blaming him for the family's woes.
In the meantime, Walter is experiencing his own internal conflict about what to do to make up for the loss of the insurance money that has been stolen by Willie Harris. After much contemplation, he has decided that life is divided up "between the takers and the 'tooken,'" and that "people like Willie Harris, they don't never get 'tooken.'" He doesn't want to be "tooken" any more, either - so, he has decided not to worry so much about what's right and what's wrong anymore. He will just act out of self-interest.
After sharing these thoughts with Mama and the rest of the family, he announces his plans "to put on a show for the man" - meaning, he has called Mr. Lindner and plans to "take" as much money from him as he can in order to keep them from moving into the house in the White neighborhood. Though doing this is basically giving up all his pride and dignity, Walter is ready to take this step despite his family's shock and dismay.
Here, the extrenal conflict with his sister reaches its peak, and she says in regards to Walter:
Oh God! Where is the bottom! Where is the real honest-to-God bottom so he can't go any farther?... That is not a man. That is nothing but a toothless rat.
Mama helps to resolve both of these conflicts when, first, she confronts Beneatha about the need to LOVE people when they need it most, not just when they make life easy for you; and she forces Walter to rediscover his pride by having Travis stay in the room when Mr. Linder arrives. Wanting to retain his son's admiration and respect, Walter takes a stand, and says the family will move into their new house. They don't want Mr. Lindner's money.
When speaking to Mr. Lindner, Walter finally expresses pride in his sister's decision to be a doctor, validating her for the first time and helping to heal the rift in their relationship. Beneatha responds with her own words of "praise" when, after Mr. Lindner asks once again if they really plan to move in, she says: "That's what the man said."
The future for the family is still uncertain as the play ends, but Walter's conflict with himself finds a resolution, and he finally "(comes) into his manhood" in the play's final scene. His conflict with his sister, too, is resolved as they find the love and respect they need to support each other whatever the future holds.