I need to separate the administration of the master and slave sites. Basically I have to move the slave sites to become independent again. What you you recommend`?

The procedure to convert a slave into a standalone joomla consists in the backup of the master that contain the PHP code and restore it on your new server or new location.

When restored in a standalone location, uninstall JMS multisites patches and JMS multisites itself.
This will give you a standalone joomla.
After that you can reuse the slave site configuration.php files that you had in the slave site deploy directory to re-use it in the new standalone.
If you are on the same server, you can perhaps keep the DB connection like it was when working as a slave.

Check also your potential specific directories like /images and /templates that you perhaps also need to restore from the slave site deployed directory.