Why did some medieval Christians connect indulgences with monetary donations?

The connection between indulgences and monetary donations in the medieval Roman Catholic Church arose primarily from the economic phenomenon of commutation. During the economic resurgence of Europe starting in the 11th century, it became common practice to convert various services, obligations, or goods into corresponding monetary payments.1

As the Church developed its penitential system, believers sought ways to perform "good works"—such as pilgrimages or charitable acts—to reduce the temporal punishment for sin, a process closely linked to the belief in purgatory. When individuals were unable to perform physical acts like traveling to Jerusalem, they questioned whether they could substitute these with equivalent financial offerings to charitable enterprises, such as building cathedrals or leprosariums.1

Church authorities and popes, including Innocent III, permitted and encouraged this commutation. Over time, this practice became increasingly associated with the "sale" of indulgences, leading to widespread criticism and abuse by pardoners, which eventually became a central point of contention during the Reformation.1