The Catechism of the Church States
"The 'treasury of the Church' is the infinite value, which can never be exhausted, which Christ's merits have before God.
The idea that a Saint Merits anything on there own is the issue here. Many outside the Church believe for example that a Catholic Christian does some kind of work to earn Merit for salvation. Merit however has more to do with Penance. To understand fully how the Sacrament of Reconciliation works can be found in the CCC.
All works done by the Saints or Christians (Catholics) are done united to the Merit of Christ on the Cross, which is infinite.
Now I rejoice in my sufferings for your sake, and in my flesh I complete what is lacking in Christ's afflictions for the sake of his body, that is, the Church
There is of course nothing Lacking in Christ afflictions, He has completed the redemption of the world, every living person past present and future. What is lacking is not of Christ but of our participation in that redemptive work. Participation means of course, things that we "do". A person might say "There is nothing you can do to be saved" and strictly speaking there is nothing (Strictly) that we can do. Then a Catholic Christian could say biblically, "That I must do(Condignly) this is or that in union with Christ in order to be saved"
Treasures in Heaven
“Do not store up for yourselves treasures on earth, where moths and vermin destroy, and where thieves break in and steal. But store up for yourselves treasures in heaven, where moths and vermin do not destroy, and where thieves do not break in and steal. For where your treasure is, there your heart will be also.
Our Merit is Condign merit not strict merit. United to Christ our merit is pleasing to God.
So, the difficulty is here is in separating the our works from that of Christ, Works done for our own Glory, is like filthy rags, but work done in union with Christ is treasure. There is no end to that merit once united to the work on the Cross.