You can see the letter here, at Tessa’s blog.
Or you can believe me when I tell you that it’s a brief, hand-scrawled note from 13 April 1973, saying that his “fiancee” (Tessa) helped to a “great extent” in the writing of the novel A Scanner Darkly, and that he owes her half of all income derived from it. He signed the note. His signature reads “Philip K. Dick.”
Tessa is selling off the letter because she’s broke. She says it hurts her to sell it, but “I am desperate, and it will not put food on the table or pay the rent.”