TR_m2_Roljir can be persuaded to release Rethyn Verelnim (using the persuasion mechanic which is cool and good) however, this can only happen if you persuade him after asking him to release Rethyn. If you get his disposition to 100 before asking Roljir to release his prisoner, he will say he's not convinced. At this point it's not very obvious that he needs persuading given that his disposition is already maxed out.

Adding a 100 disposition check that allows the persuasion mechanic to be bypassed would make sense.