The marriage of Christina Grant and John McPherson was documented in parish records my father researched in 1970s to 80s. He travelled all over Scotland and England visiting each parish. This was pre- internet days !
Statutory civil registration began in Scotland on 1 January 1855, and the legislation that established it also required every Church of Scotland parish to hand over its parish registers to the Registrar General for Scotland for safe keeping. The Aberlour Church of Scotland parish register is included among these, as you can see from
https://www.nrscotland.gov.uk/files//research/list-of-oprs/detailed-list-of-old-parochial-registers-of-scotland.pdf which was published in 1872. These are the registers that form the basis of the church records on the Scotland's People web site.
Therefore if your father found a record of the marriage locally it cannot have been in the Church of Scotland registers, because by the time he was searching locally the Church of Scotland registers had been in Edinburgh for over 100 years.
The marriage isn't listed on the Scotland's People web site in the Church of Scotland records, the Roman Catholic records or in the assorted dissenting churches' records. It's not listed in
http://libindx.moray.gov.uk/mainmenu.asp, which includes an index to the surviving Episcopalian* registers in the present-day area of Moray, which includes the part of Banffshire where Aberlour is situated. It's not in the International Genealogical Index, which is an index compiled by the Mormon Church that includes among others the births and marriages in the Aberlour parish register.
*The Episcopal Church of St Margaret in Aberlour was built in 1875-1877 and the article on Aberlour in the Statistical Account of Scotland, written in 1845, does not mention any religious denomination other than the Church of Scotland. As the brief for the articles included a specific question about how many Episcopalians were in the parish, it is reasonable to suppose that in 1845 there were few or indeed none. See
https://stataccscot.edina.ac.uk/static/statacc/dist/essay/sjsquestionsSo there are three possibilities.
First, the record is there in either the Church of Scotland or the Roman Catholic or dissenting churches' parish registers but has been omitted from both the Scotland's People index and the Mormons' index. That's simple enough to check because it's easy to get a look at a microfilm of the Aberlour parish register, or to inspect the various digitised registers in a Scotland's People Centre.
Second, your father did not in fact find a documentary record of the marriage and had to estimate the date of the marriage. I'm afraid that I think this is the most likely explanation, especially since he only gives a year, not a specific date within that year. (Nothing wrong with estimating a date - I have to do it all the time because of the gaps in the records.)
Third, and this is why I am asking, your father found a register of some kind that has escaped the attention of the Church of Scotland, the Registrar General for Scotland and the Moray Local Heritage Centre, and if that is the case I'd like to know what it is and where he found it so that I know about it for future reference and can refer to it in the hope of filling in some of the gaps and estimates in my own tree.