Can't enable dev module from Drush
You need to clear drush cache.  I have seen this same thing before, where I try to do a drush action from a module and am told the action is not available.  Then a drush pm-list | grep "module" shows me it is enabled.

drush cc drush
drush en sar


As we say around my office daily, "When in doubt, clear cache."  If this is not working, then take a step back and verify your assumptions.

We assume the download succeeded, so we should be able to find the module in a "not installed" state.  Verify with drush:

drush pm-list | grep "sar"


We assume drush handled the download properly, try again with another folder.  Verify with drush:

drush dl sar --destination="sites/all/modules"


Solution: 

I assumed, since you were asking about enabling, that this was a site module.  After downloading it on my local system I see that this is a drush command package, and that [user]/.drush/sar is correct.  Verify the module does not need enabled and is already working with:

drush help sar