فَاقْطَعُوا أَيْدِيَهُمَا جَزَاءً بِمَا كَسَبَا
The cited verse prescribes the penalty as recompense for what the two offenders have earned. (Qurʾān 5:38, excerpt)
The expression mā kasabā is treated as having a general scope: the relative mā can cover what falls within the described wrongful conduct. The juristic argument draws a subsidiary legal conclusion from this breadth.
The issue is not how to carry out a penalty. It is the classical legal question of whether a separate financial obligation for stolen property remains in addition to the prescribed punishment.