If you get anything from this site, take note of this If who ever handles your marketing AND does not cross reference your analytics, it is your best interest to disengage as soon as possible.
Your existing website has traffic, there are conversions that stem from the site. If Party B does not know what is the cause of these conversions, they may be killing the goose that lays the eggs.
In order to monitor ROAS, analytics is crucial, in order to create strategic trajectories, transparency on existing efforts is paramount, without taking this into consideration, you are driving blind. With some companies, this is not an issue, but this is not a typical approach to marketing, and certainly not the methods we gravitate towards