In 1875 he paid £105 10s in rent (1875 Valuation Roll - ScotlandsPeople).
No, that's not what it means.
The so-called rentals on a valuation roll are an assessment on a rather arbitrary set of criteria. This so-called rental value is then used to calculate the amount of money payable in rates (i.e. local taxes) to the local council each year.
Whatever rent he actually paid could be quite different, and the only place you would be able to find that that would be in the records of the estate that owned the property. I do not know if the estate records of Sir William Gordon Gordon Cumming have survived or if so whether they are accessible.
You also need to be sure that you don't confuse Sweethillock in the parish of Drainie with Sweethillock in the parish of Alves. These are two entirely different farms.
One in the parish of Drainie
https://www.geograph.org.uk/gridref/NJ1969One in the parish of Alves
https://www.geograph.org.uk/gridref/NJ1361J Robert Walker would be a Writer, not Whiter. A Writer is another word for a lawyer or solicitor.