There is an Alex Davis on the 1870 Comal Co, TX census, but he is listed "M" for mulatto.Your David is listed "W" for white.
1880 census Menard Co, TX:
Category: Name;Relationship to head of household; Marital Status; Gender; Race; Age; Birthplace; Occupation; Father's Birthplace; Mother's Birthplace.
David DAVISSelfMMaleW52ALCarpenterMIMI Emilia DAVISWifeMFemaleW42ALKeeping HouseNCGA Martha DAVISDauSFemaleW13TXALAL Josian DAVISSonSMaleW11TXALAL Jefferson DAVISSonSMaleW9TXALAL Alexander DAVISSonSMaleW7TXALAL Thomas DAVISSonSMaleW2TXALAL
On the 1870 census David is listed as renting, not owning land.And, unfortunately, the 1880 census does not ask if one owns land.I would still check land and tax records in Menard Co, TX. If he died there, and did own land, his heirs will have to sign off to give clear title to the buyer.
I checked the handwritten record for the 1880 census and David's parents are listed as born in Miss.Not MI as shown on the FamilySearch site version of the census.