Earlier quoted context omitted.
Well, if we're discussing whose data it is the information about how much I pay you , even from a devi's advocate perspective, you can't do better than arguing that this data pertains to both of us. So we should share the property of that data somehow. I don't see how you could argue that that data would be solely the employer's data.
If I administer a survey, collect responses, and put them into a spreadsheet, is the data in that spreadsheet not mine despite the fact that it consists of things that other people told me? I can't share it without the permission of those surveyed, assuming I didn't promise not to?
So while, in many jurisdictions, it's fine for companies to sell data collected on their employees, and it could be argued that those employees consented to this data sharing by working there, one could also easily argue for an employee protection law that prevents companies from requiring their employees to consent to this.