On the
JobRoller demo you can see the company name and "posted by" link on the main index and single page listings for each job. The company name link goes to the company's external website, and the "posted by" link takes you to the employers profile and listings page within JobRoller. Are you saying you don't want the link to the job to go to the JobRoller ad listing, but instead to the actual external website page? If so, this essentially would effect the whole job listing process, as presumably you'd also want the job listing form to be on the external site? This is also the case with the existing "Apply Online" link, if you wanted this to redirect to an application form on the external site, there would need to be customization of the theme involved so that everything redirects differently to how it is actually built into the theme.
Ultimately this could technically be done with customization of the default theme on your part, however the payment process should also be considered as I imagine this aspect would not be as straightforward as simply changing some links. Presumably the theme may not recognize the payment functionality unless it's also modified to accommodate the changes.
Hopefully I have understood your requirements correctly, but if I'm off track here, please feel free to clarify further. Thanks.