if its a quality website then its no prob. but one thing for sure too much external link will harm your rank in google
Google is not allow to buy or sell links, so may be in Google your site will penalize, but in adsense you will never penalized...
If you add a link to the site that is relevant to yours it should be fine. Make sure it is placed organically on your site though and certainly on one page only (not sitewide)