You should probably try to cabal install the package that actually depends on persistent-postgresql
as cabal will then try to select the right version automatically.
However if you do need to do this, a simple way is:
cabal install persistent-postgresql-1.2.1.3
Or more complicated:
cabal install persistent-postgresql --constraint 'persistent-postgresql < 1.3'
Here you can just keep specifying that you want something earlier than a particular version as things don't work without needing to know the exact next version down. You can also place constraints on other packages if necessary.