This portion from the Psalms unequivocally indicates the impossibility of sinful man making any ransom for sin (which includes the guilt of temporal sin):
"Even those who trust in their wealth and boast in the abundance of their riches? No man can by any means redeem his brother or give to God a ransom for him—For the redemption of his soul is costly, and he should cease trying forever—That he should live on eternally, that he should not undergo decay." (Psalm 49:6-9)
The concept of indulgences involves shortening the time that a deceased person must spend in purgatory through performing various meritorious deeds, which are bestowed grace from the treasury of merit drawn from Jesus Christ, Mary, and the saints. But Psalm 49:6-9, which speaks specifically of human death as being an unavoidable consequence of the fall, affirms that no man can offer any price to God to make any type of restitution. We cannot offer even the smallest amount.
Only God can fully pay off a debt that He requires (Psalm 49:15). No man can make reparation for his own sin or the sins of other people, which is essentially what takes place in purgatory. Indulgences are of no avail. Only Jesus Christ could take on the task of satisfying the guilt of sin. We cannot do that even to the smallest degree.