Okay, I figured this out based upon suggestions from StackOverflow. This is how I resolved this issue:
I created a group called ftpusers.
groupadd ftpusers
I added my wordpress user to this group.
usermod -a -G ftpusers wordpressuser
I changed ownership of the wordpress folder recursively to my wordpress user:group.
chown -R wordpressuser:ftpusers wordpress
Voila — my package installed.