There were two William Fosters in Claiborne Parish in 1850 that I am aware of. One of them was my ggrandfather. The other one was 30 years old with wife Sarah and daughter Ann and son David. I don't know the descendancy of this William, but it may give you a point to start. You can find this William on p94, 1st Ward, Twp 24, household #16. Good luck.