I think you can put it in there tonight, with a slow acclimatation.
Then you will do 10% water chages every day...
On another note: external filters need to be cleaned now and then. You must do that one day...it is cycled and mature, of course, but it also is full of ****...
Non mantained cannisters a nitrate factories.
Edit: i misread. Sorry. You bought the filter and brought it full. Sorry about advising you to clean the filter....
How long was the journey with the filter full?
Do a water change again tonight, to wash out eventual dead stuff that was released back into the water.
Can you do a quick cleaning of the material in the filter tonight, with tank water, just to take out the gunk?
By tomorrow morning i would put the beastie in.