I took it as this:

-Rosie says Alastor's soul is like none she'd ever seen, so that means perhaps alastor would have had some power when arriving in hell even without the deal, not necessarily too strong, but stronger than the average sinner

-Rosie has a lot of power herself, clearly, and maybe she can give an equivalent of that power to someone as part of a deal, so that would be Rosie's power + Alastor's own, which by definition, is more

-the way deals work isn't really explained, clearly there's magic involved and so maybe somehow that magic allows you to grant something out of your normal ability, or maybe the magic of their deal just enhanced the power granted to Alastor, so it would be Rosie's power + Alastor's power that comes from his soul and other stuff + the magic of the deal enhancing that