«بِعْتُ مِنْكَ هَذَا الْعَبْدَ بِكُرٍّ مِنَ الْحِنْطَةِ»
“I sold this slave to you for a measure of wheat.”
«بِعْتُ مِنْكَ كُرًّا مِنَ الْحِنْطَةِ»
“I sold you a measure of wheat.”
In the first formula ba assigns the wheat the role of consideration for the article sold. In the second, wheat itself is the direct object of sale. The rules concerning the contract and substitution before possession can therefore differ.
The difference between “sold for wheat” and “sold wheat” is not merely stylistic: it identifies different legal roles of the two items.