Here is the sentence:
A person who agrees to serve as mediator between two warring factions at the request of both abandons by so agreeing the right to take sides later. To take sides at a later point would be to suggest that the earlier presumptive impartiality was a sham.
I don't understand the meaning of "by so" in this sentences. Does the sentence mean "by abandoning agreeing the right to take sides later, a person can become a mediator", or "by serving as mediator, a person must abandon taking sides later"? What does the "so" refer to? And is "by so" a "adverbial modifier" of "abandon agreeing"?