In a sale, ba associates the item sold with its price or countervalue. The textbook uses a specified measure of grain in a historical transaction.
«اشتريت منك هذا العبد بكرّ من حنطة جيدة»
I bought this servant from you for a measured quantity of good wheat.
Here kurr is a grain measure, and ba connects the object of sale to its consideration. The text links this construction to the treatment of price (thaman) and substitution. For a neutral modern parallel: “I bought the book for five dirhams.”