Ritual Deferral

Not sure if is a bug or I don't fully understand the mechanic. Here is the issue, I have the deferral cost reduction talent Promised Devotion. I try to defer an item, this are the stats of the deferral process:

Future Discount 484.
Deferral fee is 0.
Total Deferral Cost 366.

Currently I have 433 Tribute, enough to pay the total deferral cost. However, I can't defer the item. Seems that you need to have at least as many tribute than the future discount to be able to defer the item, even tho the total cost is lower. Not sure if is working as intended and total cost is not the actual total cost or is a bug.