This is a state subsidy for clerical salaries in Austria. It was meant for the working clergy, and comes from the 18th-century sale of lands held by the "idle" monks.
(Latin for "suitable") The congrua is an Austrian state subsidy to bring clerical salaries up to a “suitable” standard. The church defines it as "the lowest sum proper for the yearly income of a cleric". Congrua are mentioned explicitly in the Austrian Concordat of 1933 (Art. 15.3) and described in the Italian (Lateran) Concordat of 1929 (Art. 30): "The Italian State […] shall continue to supply the deficiencies in the income of Ecclesiastical benefices […]”. Such payments are justified as being restitution for state confiscation of Church property in past centuries. (Naturally, no mention is made of the fact that the Church obtained this property by frightening the dying with tales of Eternal Torture....)
E.g., The Austrian Concordat (1933, Art. 12.2) stipulates that the congrua could be drawn from the "religion funds" (administered by the state).
What is the origin of these Austrian "religion funds"? They go back to the 18th-century Holy Roman Emperor Joseph II. He was an “enlightened despot”, whose motto was "Everything for the people, but nothing by the people."
He tried to help the Church do good by supporting its ministry and its social services — and eliminating the rest. Thus he said that no one should have to go more than an hour by foot to attend church and he established a thick network of parish churches to accomplish this. He also founded seminaries for priests to improve their ability to serve their flocks. And any monastic orders that ran schools or hospitals he left in peace to get on with the job, but the rest he expropriated. In one stroke his Edict on Idle Institutions more than halved the number of monks in his empire. The proceeds from this formed the religion funds from which, to this day, are drawn the congrua to pay the salaries of Austrian priests.