The meaning of "by so" in the sentence [closed]

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"?

It's parsed: [by][so agreeing] instead of [by so][agreeing]

The essence of the sentence is:

A person who agrees to serve, abandons (gives up) the right to take sides.

You can substitute "by agreeing in this way" for "by so agreeing":

A person who agrees to serve as mediator between two warring factions at the request of both, abandons (by agreeing in this way) the right to take sides later.