This letter is not actually seen in The Price of Neutrality. It is only read by Geralt when Merwin Ademeyn shows it to the witcher as evidence that he and his party have every right to be at Kaer Morhen. This is a rather obviously point of disagreement between the witchers and the hunting party.
Letter of safe conduct
Let it be known far, wide and by all, that I hereby declare Merwin of Caingorn to be our loyal and trusted servant, and to be loved by us, thus rendering all who cause him greef liable to incur our Royal wrath.
This is a nod to one of the first short stories about the witcher, "The Lesser Evil". In that tale, Renfri had a letter of safe conduct which she claimed was signed by King Audoen and bore his seal, asserting that she and her band were working with the his knowledge and blessing. Suspiciously, the letter misspelled the word "mistreat" (mistreet), but Renfri quickly glossed over this.