It is possible to change the return URL. You can hook into the partially_gateway_settings filter and override the payment_plan.meta.checkout_cancel_url property of the array. Here’s an example of how you could do that by adding a function to your theme’s functions.php file
function custom_partially_cancel_url($config) {
$config['payment_plan']['meta']['checkout_cancel_url'] = 'https://example.com/url';
return $config;
}
add_filter('partially_gateway_settings', 'custom_partially_cancel_url');