Since the Boadacean Traitors' Guild has killed Da-Ud's half-sister while she was with Simon, both have a motive to cheat the Guild but how is Simon's deception meant to accomplish this? Using the Rood-Prince's "...toposcope-scriber..." and "...brain-dictation laboratory..." (I am not sure what these are), he generates documents purporting to show that he has sold to Da-Ud partial evidence that High Earth has conspired against several major human powers "'...for purposes of gaining altitude with the Green Exarch." (Anywhen, New York, 1970, pp. 33, 34) Both High Earth and the Exarch would pay for such documents. Da-Ud is to tell the Guild that the remaining evidence is also for sale at a higher price. However, this is a "...double blind..." (p. 35)
When Da-Ud has dealt with the Guild and suggests that Simon directly sell them the remaining evidence instead of cheating them, Simon senses that Da-Ud is helping the Guild against him. He proposes to sell the remaining evidence to the Rood-Prince so that the Guild can acquire it by attacking the Principality, thus saving Simon from assassination by a hypothetical second agent from Earth if he is seen to approach the Guild directly with the information.
When Simon, having been captured, is interrogated by the Traitor-in-Chief of Boadacea, he informs the latter that the apparently human representative of the Exarch who is with him is in fact a vombis, a metamorph entirely loyal to the Exarch. Then he turns his coat under Guild laws, offering the information which was not found in the attack on the Principality but to Boadacea, not to the Exarch. His stated motive is that he wants to deal fairly with Boadacea, then become honest with himself. His offer accepted, he writes the requested document, whose authenticity can be confirmed toposcopically, and adds an alliance with High Earth, which was his object from the start.
An alliance with High Earth would not have been acceptable as long as Boadacea was linked to the Exarch but Simon has severed this link by revealing the identity of the vombis and by the impressive elegance of his surrender. Does this make sense?