According to documentation, FindAndModify method should return original document by default.

But, it seems like, without specifying VersionReturned it returns modified document.
I looked at the MongoCollection class (in github) and at the line #560 it does a check. And, because the default value of VersionReturned property is null, then condition returns false. Therefore, Core.Operations.ReturnDocument.After will be used.

It case of using the following overloading of method:

public virtual FindAndModifyResult FindAndModify(IMongoQuery query, IMongoSortBy sortBy, IMongoUpdate update)
{
    var args = new FindAndModifyArgs { Query = query, SortBy = sortBy, Update = update };
    return FindAndModify(args);
}

It doesn't specify which version of document should be returned.

According to this fragment:

var returnDocument = args.VersionReturned == FindAndModifyDocumentVersion.Original
    ? Core.Operations.ReturnDocument.Before
    : Core.Operations.ReturnDocument.After;

It will be the same as:

var returnDocument = (FindAndModifyDocumentVersion?)null == FindAndModifyDocumentVersion.Original
    ? Core.Operations.ReturnDocument.Before
    : Core.Operations.ReturnDocument.After;

Need to add checking for null value and handle null as a default (original\before document) value.