One principal juristic function of ala is to place something under a person’s obligation or liability. It is therefore significant in financial acknowledgement.
«على: للإلزام»
Ala expresses obligation or liability.
«لفلان علي ألف»
I owe so-and-so one thousand.
The speaker acknowledges an obligation owed to the other person. A mechanically literal translation such as “upon me” may conceal the legal force of the wording.