Flag on the play! NFS between Mac and Linux just doesn't work. I tried and tried and tried and never could get it to go. Google the topic. You will find that this is a celebrated issue. There are LOTs of programs out there for your Mac that will tell you that they will configure your Mac to make this work, but it just doesn't. Stay away.
Instead, you may need to view this another way. Instead of the Mac mounting the Linux box and pushing the files for a backup, what if your Linux box mounted the Mac file system and pulled the files for the backup? Then you could use a cron job on the Linux side to automate this.
The secret sauce is the sshfs, a user space file system (need FUSE to be loaded - usually a simple kernel module you can modprobe) that works on top of ssh. Happily, your Mac come with and ssh server, so you are set. This is what I do, and freely pull files off my Mac and onto my Linux system.
If you are running KDE, you can even bypass sshfs, if you want to do things manually. Start up Konqueror, switch it to two pane mode, and open one of the panes on your Linux file system. For the other, go the address line and type in:
fish://ip-address-of-your-mac
This oddly named approach invokes a built in SSH file system capability and will prompt you for your Mac userid and password. Once entered and accepted, the Konqueror pane will be viewing your Mac file system. Drag and drop to your hearts content!