According to Caridnal John Newman in his Anglican days, anyway, the doctrine of treasure of merits is as old as the Creed:
27. [The main feature in modern Catholic teaching, as distinct from that of Antiquity, is the doctrine of the "Treasure of Merits," but the thing is in the Fathers, though not the phrase.This doctrine is founded on the article of the creed, the Communion of Saints, according to which the Christian body is like an expedition of pilgrims, helping each other with all their powers and in every way by temporal aid and spiritual, with prayers, good works, sufferings, as they go forward towards heaven, and that, up to the hour of death, when each shall stand by himself and "bear his own burden."
Beginning with this great doctrine, we teach that the Church has the prerogative of effecting the remission, in whole or part in each case, of such punishments as are still due for venial sin or for forgiven mortal sin, not only by the Eucharistic Sacrifice, &c., but also by setting against them, or rather, pleading with God, that infinite treasure of merits which our Lord has wrought out, first in His own Person, next through the grace which He has given to His saints. I say, "next," for this treasure consists essentially of His own merits, not of His Saints'; and includes theirs, only as it includes also those of good men on earth. Moreover, its benefits cannot be given in any measure, great or small, except in regard of the punishment of past sins, already repented of and forgiven.]
(footnotes to On the Prophetical Office of the Church: Vol 1)
As far as I can tell, the first time the Treasury of Merits appears in writing is in Clement VI's bull of 1343. I haven't read it, so I don't know what is said about it in that document other than the granting of indulgences. But it may be something similar to the line of argument that Newman takes.