$partnerTitle = $pages->find("parent=1004,partnerID=$partnerID")->title;
Will not work cause find() returns a page array and not a single page.
Rather use
$partnerTitle = $pages->get("parent=1004,partnerID=$partnerID")->title;
Or even
$partnerPage = $pages->get("parent=1004,partnerID=$partnerID");
$partnerTitle = $partnerPage->id ? $partnerPage->title : "PartnerPageNotFound";
But why you need a function for that is beyond me. And also why there's obviously an ID you enter manually to then get a page? Why not use a page field to select the partner page?