Objection 1. It would seem that an indulgence can sometimes be granted to one who does not fulfill the required conditions. Because when a person is unable to perform a certain action his will is taken for the deed. Now sometimes an indulgence is to be gained by giving an alms, which a poor man is unable to do, though he would do so willingly. Therefore he can gain the indulgence.
Objection 2. Further, one man can make satisfaction for another. Now an indulgence is directed to the remission of punishment, just as satisfaction is. Therefore one man can gain an indulgence for another; and so a man can gain an indulgence without doing that for which the indulgence is given.
On the contrary, If the cause is removed, the effect is removed. If therefore a person fails to do that for which an indulgence is granted, and which is the cause of the indulgence, he does not gain the indulgence.
I answer that, Failing the condition of a grant, no grant ensues. Hence, as an indulgence is granted on the condition that a person does or gives a certain thing, if he fails in this, he does not gain the indulgence.
Reply to Objection 1. This is true of the essential reward, but not of certain accidental rewards, such as the remission of punishment and the like.
Reply to Objection 2. A person can by his intention apply his own action to whomever he lists, and so he can make satisfaction for whomever he chooses. On the other hand, an indulgence cannot be applied to someone, except in accordance with the intention of the grantor. Hence, since he applies it to the doer or giver of a particular action or thing, the doer cannot transfer this intention to another. If, however, the indulgence were expressed thus: "Whosoever does this, or for whomsoever this is done, shall gain so much indulgence," it would avail the person for whom it is done. Nor would the person who does this action, give the indulgence to another, but he who grants the indulgence in this form.
Source: Summa Theologica (New Advent)