Appstore does not save partially downloaded file
You should use wget/ curl based methods and Apple Developer website for downloading it. I am not a fan of App Store managing huge downloads. Since I am not doing iOS development, I can work with old version too, so don't need App Store to recognise the download and give me an update.
Can't update Xcode
Download older version of Xcode?
I don't know about CURL, but in wget you can do
wget -c https://site
-c lets you continue the download where you started.
Reclaim the space using apps listed here:
- How can I figure out what's slowly eating my HD space?