You can't rent extra space on your iPhone, so perhaps you're referring to the iCloud synchronization service that makes all the Photos Libraries on connected devices have exactly the same pictures. So, as the word "synchronize" implies, you can't have more pictures in one Library than another, and still be synchronized-- this is, and still use iCloud Photos.
One thing you can do to relieve some space while still being in sync is to to use Optimize Storage. You can use "Optimize Storage" on any device, independent of the others. So you can have "Optimize Storage" on your phone but not your iPad, if you like. On an iPhone, you set "Optimize iPhone Storage" in Settings>Photos. (In iOS 18, it's Settings>apps>Photos, and it may be slightly different in other OSs.)
If you set a device to "Optimize Storage," then Photos may store only smaller images on the device and rely on iCloud to keep the full sized images. So, if Optimize is chosen, and you want to magnify a picture to see more detail, Photos will reach out to iCloud to get a full sized image for you to zoom in on. It's the same for editing or printing or anything that demands the full picture. Your optimized Library may take up less than 20% of the space of a fully downloaded Library.
If you already have "Optimize Storage" checked on your iPhone, and you're still running out of space, then there may be other (more awkward) solutions. Let us know, and include information about the OS and model of your devices, and the used and free storage space …