In one of the Aubrey-Maturin novels by Patrick O'Brian, the eponymous Captain Aubrey, then in command of a Royal Navy frigate in the Indian Ocean sometime in 1812 or 1813, receives a letter telling him that two "livings" have fallen vacant and that he is supposed to appoint an "idoneous" person to fill them. Aubrey is able to exactly predict the "tithe" and is evidently free to choose any Anglican clergyman to fill the positions (both of which he offers to one Nathaniel Martin, who is also on the ship as a sort of unofficial assistant surgeon).
What kind of system was it that gave rise to this? Why would Aubrey have the authority to decide who should be the parson of which place, and how was it established exactly how much that parson would get? Was this actually the way the Church of England was organized?
If it matters, the Aubreys were landed gentry and Aubrey was also MP for a rotten borough.
I'm not familiar with the novels but I can take you through the terms involved and the systems that they refer to.
A church and it's parish is also known as a "benifice", normally this refers to a single church although in modern times we see "multi-benifices" as clergy numbers and congregations drop.
Being the vicar of an Anglican church was known as having or holding the "living": receiving payment from the benefice in return for dispensing religious duties in the parish. The money for the parish came from "tithes", roughly 10% of each parish adult's earnings. Many older towns and villages still have street names which refer to the locations where these were collected, e.g. Tithe Lane or The Tithe Barn. If one knew the general income of a parish then one could adequately anticipate the corresponding tithe amount.
As you can imagine the collection of tithes could be very profitable, depending on the area, so the "livings" of churches were tradeable commodities. For example, the living of St. Patrick's church in Patrington, East Yorkshire, was owned by Clare College, Cambridge, who thereby held the advowson to nominate vicars. Naturally Clare College would also be in receipt of a portion of the tithes collected in Patrington.
In the novel it seems that Aubrey, or a party with whom he was associated, held the living of the parish and its advowson. It would be up to that party to decide how much the vicar would earn although records suggest that an unscrupulous incumbent could easily un-balance the books to line their own pockets. It would be hoped that an "idoneous" person, i.e. somebody fit and proper, would take the living but that was not always the case.
Advowsons were normally held by senior figures in society such as landed gentry so it doesn't surprise me that your Aubrey was such a person or that he associated with such people.
Sources:
A Seventeenth Century Attempt to Purify the Anglican Church, Calder I M, 1948
The Church of England, 1688-1832, Gibson W, 2001
Advowsons and Private Patronage, Sutton T, 2019