Apache :: FTP Accounts And User Permission?

My apache VPS is having a minor problem.

Using SSH, I gave permission to my FTP username with code below:
chown -R user:ftpgroup /home/site

But my WP theme needs to set up that user I use is rewritable and use this code:
chown -R www-data:www-data /home/site

since using a php code to check WP user and it shows www-data.

MY PROBLEM: I need my FTP to able to write cause i use plugin to create backup and change plugin. My theme also need for both to work.

I can't seem to make run together. I can change group or FTP as long as I can have m WP working.