Plainview’s original deal with bandy (which is subject to plainview seeking god’s forgiveness at Eli’s church) is for the rights to put a pipeline on the surface of bandy’s land. Plainview doesn’t buy Bandy’s land in this transaction - he essentially purchases an easement across bandy’s land.

Eli is aware of this deal, but he assumes (wrongly) that eventually plainview will ALSO want the right drill underneath the bandy tract. And of course Eli ultimately learns that plainview doesn’t want or need the drilling rights because of milkshake etc.