I'm afraid I don't know who your Willie Marshall Rigsby was. I have checked all my notes and can't match him up with anyone.
There was no Willie or William Rigsby in Dallas Co AL in 1870. The only William + Nancy is in Bledsoe Co TN, and that was William Glenn Rigsby, who was born about 1837 and did not have a daughter Martha.
I can't find a Martha under age 1 anywhere in 1870, and I can't find a Martha around age 10 anywhere in 1880.
Message 218 on this forum says that William who married Nancy Miller died at age 45, so he must have been born about 1828. I checked the 1850 census, and there are 5 Williams about the right age. Three of them I can identify living to 1880, but the other two I lose track of.
I think the better chance is this one:
1850 census Cobb Co GA p98 Allen Rigsby 45 GA Mary 49 NC Bethina? 22 GA William 20 GA John W 18 GA Mary C 15 GA Martha 13 GA
I haven't found this William after 1850. His father Allen was a son of Allen Rigsby + Margaret Johnson. There is a book called "Rigsby Relatives and Related Families" by Margie Milner Boyd that deals with Allen + Margaret's descendants. It may say what happened to this William. I don't have access to a copy, so can't check.
The other possibility is this one:
1850 census Carroll Co TN p74a Elizabeth Rigsby 45 NC James A 23 TN William A 21 TN John H 20 TN Paralee M 16 TN Moses 13 TN Francis M 10 TN Daniel S 6 TN
I haven't yet figured out who Elizabeth's husband was. This William is still living with his mother in 1860 in Carroll Co, but I can't find him after that.
I need to go back and check the 1870 census for AL. There are some counties where I didn't find people I expected. Maybe your William will turn up.
Sorry I couldn't give you any definite help. Let me know if you find anything more about William. I'd like to know how he fits in with the rest of the Rigsbys.