Where do you get the verb "roams"? One can supply verbs at times in Latin when the yare not given, but usually it is the verb "to be."
Also this cannot be the case because "dead dog" cannot be the subject. That would like like canis mortuus; here instead we have the direct object, so it must receive the action of the verb. SO your translation doesn't work.
I know plenty of latin phrases, here's an example: about Sir. Isaac Newton
'Qui genus humanum ingenio superavit'
if you translate it
'in intellect he surpassed the human race'
but a loosely translation is
'Of all humans, there is no greater intellect'
you just cannot traslate 'canem mortuum liber' word by word and expect it to make any sense. That's not how it works my friend.
I'm 99% sure the loosely translation is "the dead dog roams free again" ( spectral hound?, liber? just common sense applied here )
here's another one:
"Ad astra per aspera"
"Through hardships to the stars"
or more loose
"A rough road leads to the stars"
dont tell me: where did they get 'road" or "leads" from?, again, that's not how it works
u need to do a little more research
EDIT. If someone studied languages and can point my errors. I would appreciate that