I've misread the OP.
The problem I've to solve is similar, but is already at group level.
This is a simplified exemple:
Let say I have GroupA and GroupB with the following folder structure
RootFolder
|- Folder 1
|- Folder 2
|– Folder 2.1
|– Folder 2.2
If I want GroupA and GroupB to be able to edit all that structure except "Folder 2.1" where I only want to grant access to GroupA.
The administrator has to:
Grant access on RootFolder to all those groups
Go to Folder 2.1 permission settings
Remove inherit permissions on this folder
Re-grant access to GroupA on this folder
This sound pretty simple, because the example is simple … but if in fact I to create this kind of structure often (I use a space template) and in each structure, I manage more groups and more exception folder … it would be far easier to just go on exception Folder, don't change inheritance, but restrict access on the folder to the 2 or 3 concerned groups.