Meaning depends on construction and subject matter. In a sale or exchange, the textbook allows ala to function like ba, expressing the price or countervalue.
«بعت هذا على ألف»
I sold this for one thousand.
The point here is not an unrelated prior debt; the clause specifies the consideration in a sale. Context supports reading ala as “for” or “in exchange for.” The legal setting helps determine the intended sense.