What does the agreement state? They need to make sure they are in compliance with the agreement for income, loss, capital accounts, and distributions (which are different than income allocation).

If partner A is in the 37% bracket and partner B is in the 0% bracket (before income allocation); shifting income to B may not be looked at kindly by the IRS.

Why would A be willing to economically give up income and the increase of capital account without compensation?

They need to discuss with a partnership expert, not just a run of the mill CPA.