«وَيُبَايِعْنَكَ عَلَى أَنْ لَا يُشْرِكْنَ بِاللَّهِ شَيْئًا»
“And they pledge allegiance to you on the condition that they associate nothing with God.”
Here ala expresses a condition. A condition is not identical to the consideration exchanged in a sale; the consequences can differ when it is not satisfied.
This matters in the statement «طَلِّقْنِي ثَلَاثًا عَلَى أَلْفٍ» (“Divorce me three times for a thousand”). On the Hanafi analysis attributed to Abu Hanifa, if only one of the requested three pronouncements occurs, the full payment does not become due, because the specified three function as the condition. A different treatment is reported from his two students.