Left- and right-dislocation in German behave differently regarding the case the dislocated expression takes. Left-dislocation seems to be lenient, as it allows the nominative as well as the case the resumptive pronoun takes:
(1) a. DER MANN, DEN kenne ich nicht. b. DEN MANN, DEN kenne ich nicht. 'The man, I don't know him.'
In (1a), the dislocated expression DER MANN appears in the nominative case, even though the resumptive pronoun DEN is in the accusative case. In (1b), the dislocated expression is in the accusative, thereby matching the case of its resumptive pronoun. I am German native speaker, and for me both (1a) and (1b) are good.
Compare these data to those of right-dislocation:
(2) a. * DEN kenne ich nicht, DER MANN. b. DEN kenne ich nicht, DEN MANN. 'I don't know him, the man.'
Example (2a), where the right-dislocated expression appears in the nominative is now ungrammatical. If the case of the right-dislocated expression matches, as in (2b), the sentence is good.
What principle could account for this asymmetry?