Not sure but I recall reading that there are two SPECIFIC souls that you need to forge into a third before you can trigger that dialog.
If your appraise isn't high enough (or is 0), just bring Gann along to bargain with the Balors, and it will be reduced to 2. The souls you can get:
Fused Soul - Soul Container 91, and 127 - Talk to Prouset to get a note, check them out, put them in forge, and get him to fuse them.
2 Unnamed Soul Container - In Teacher's Dormitary
Artesh's Soul - Get from winning side bet with final match of golem fights.
Orjul - Devour Soul on the Red Wizard student when the teacher asks you to demonstrate (you need the feat).
Note that the fused soul is traded for Ammon Jerro's soul.
Also, there are more golem parts downstairs and there were some in the Deep Imaskari areas of the game such as the Skein and the upper area, all around the Sleeping Coven basically. There are legs that give haste, and weighted arms. Use those alone with regular body and head and you can win all the fights except the last one. Switch to electric arms for the last one. Dont' forget to heal your golem between matches!
Don't know if that helps or not.
And then I found THIS:
Posted 10/01/07 21:20 (GMT) by Eric Fenstermaker
This issue was fixed on our end a few weeks ago and that fix will be available in the next patch, but as you've noticed, getting Ammon's soul before completing the Pit Fiends' soul fusion quest will indeed cause you to not be able to check out the requisite souls from the soul depository for the latter.
Simple workaround for that in the meantime - hit '~' to bring up the console, then type the following in the game:
rs ga_local_int( "nSoulsWithdrawn", "-1", "$MODULE")