I have been going through the journals of Alfred Doten, who came to California in 1849 as a prospector and stayed as a farmer; the period 1849 to 1857 so far. He listed his financial gains and expenses in detail but I can find no mention of "buck" (Sacramento newspaper 1856).
He uses dollars fractionally and decimally ("washed 180 buckets today and got 20 dollars" or "only made 4 1/2 dollars today"). His purchases are stated decimally. In one day of washing, he does add "two-bits" to his take in gold, but "buck" does not appear.

I will carry on into the 1860s of these rather voluminous diaries.

"The Journals of Alfred Doten, 1849-1903," edit. Walter Van Tilburg Clark, 1973, University of Nevada Press, Three volumes.