Actually, I'm pretty sure that if you tell him you serve Talos and are telling the truth, the pledge is indeed succesful. Though perhaps you need to actually have started the stronghold quests first? If it's not working properly, maybe it's a bug. Or perhaps my memory's just playing up, and such a thing doesn't exist at all .
Not that I've ever actually played one, but I did read that if you are an evil cleric, the quest is to steal the ring, not retrieve it. Was that wrong then? If so, you're right. It makes no sense at all.
For that matter, to my mind, neither does asking a good cleric to assassinate someone, even if the target is evil (part of Lathander temple stronghold).
Assume we've got an evil CHARNAME. He decides to accept the offer of the Talos Temple to join up after the Beholder quest. One of the quests later on is to retrieve the Lathlander's Dawn Ring from a thief who passed it on to some guy on top of the Copper Coronet.
What doesn't make sense is this: If you're working for Talos whole heartedly, how come you can't recite the 'pledge of allegiance' to Talos when asked to do so by the fence so he'll turn over the ring without a fight? I just think the original Bioware coding is stupid. You've joined the Temple so there is no reason why you can't give the fence what he wants so he turns over the ring peaceably.