What is the difference in treatment when an incoming partner purchases an interest by agreeing to perform services for the partnership and the partnership (1) gives a 25% interest in the capital of the partnership, (2) gives a 25% interest in the future income of the partnership, or (3) gives a 25% interest in the partnership's future income which the incoming partner must forfeit if the partnership, as reconstituted after the incoming partner's admission, is unable to earn an average of $100,000 profit for the ensuing five years