Q. 1. What is the difference between paying a priest "sustenance" versus "remuneration?" Are they not the same?
A. 1. To answer your question, I would like to quote a letter.
On November 8, 2013, Mons. Antonio Neri of the Congregatio Pro Clericis” at the Vatican wrote a letter to Mr. Caridi on the matter of priestly “sustenance versus remuneration.”
His letter made reference to two paragraphs in the Code of Canon Laws as follows:
Can. 230 §1. Lay men who possess the age and qualifications established by decree of the conference of bishops can be admitted on a stable basis through the prescribed liturgical rite to the ministries of lector and acolyte.
Nevertheless, the conferral of these ministries does not grant them the right to obtain support or remuneration from the Church.
281. §3. Married deacons who devote themselves completely to ecclesiastical ministry deserve remuneration by which they are able to provide for the support of themselves and their families. Those who receive remuneration by reason of a civil profession which they exercise or have exercised, however, are to take care of the needs of themselves and their families from the income derived from it.
On the aforementioned, he explained:
“Sustenance is the more fundamental of the two, providing for a cleric’s essential needs, such as food, shelter, and adequate medical care. Honest sustenance is a basic right rooted in incardination and is not dependent upon a cleric being dedicated to ministry.”
“Remuneration (cf. C. 281, §1) provides more amply for a cleric’s needs in such a way that it frees him for the work of ministry which he is about to undertake, and thus it is dependent upon his dedicating himself to sacred ministry. The right to remuneration is not absolute, and may be lost entirely under certain circumstances. Understanding the clear distinction between sustenance and remuneration can clarify many of the situations which you describe in your letter.”
So "sustenance" is a basic income that all clerics are entitled to as members of the Diocese. "Remuneration" is like a salary paid to the priests so they can minister to the faithful without having to worry about their basic needs.