Also living in 1850 Buffalo, Butler County, Pennsylvania is Jacob STALEY, age 75, farmer, born in Pennsylvania and his wife, Mary, age 62.Have you ruled out Jacob as William's father?As Jacob owned a farm worth $4000 perhaps you could find a will mentioning his children or estate record or see what became of this land.
Similarly as William owned land in 1850 you could see if his purchase of this land gives any clues to his origins.
There is an 1869 Lancaster County, Nebraska marriage for Ephraim STALEY, son of William and mother's name unknown, whose birthplace is given as Pittsburg (sic), Pa.While that may be a clue, my GUESS would be that he just named the largest city to Butler County where I'd GUESS he'd actually have been born.
There is an Ephraim STALEY who served in the Civil War in Company A of the 11rh Kansas regiment and who had a Civil War pension.This Ephraim died in Oklahoma and was born in Pennsylvania so may be from your STALEYs.A pension might also give a specific birthplace but wouldn't help with the names of William's parents.
I would try searching more for information about Samuel's siblings if you're not getting anywhere with Samuel and I'd gather all information that you can about Butler County, Pennsylvania STALEYs as eventually it may tie in.