Expiation for a broken oath illustrates how the permissible form of fulfilment can depend on a person's capacity when the duty is discharged.
Suppose financial expiation is initially possible, but before payment the person loses that capacity and becomes legally unable to afford it. The prescribed alternative of fasting then becomes relevant under its own conditions.
Certain duties thus interact with the legal status and capacity of the obligated person. They cannot always be frozen according to the first moment at which liability arose.
This is not an unrestricted choice between monetary expiation and fasting; the order and conditions of expiation remain defined by their governing texts.