Is it permissible to collect khums on behalf of the Imam in the time of Ghaybah?
I find there are Sahih traditions that say it is impermissible to use the Imams money even if for a good cause

The argument is that the Imams have given permission to qualified scholars to collect it. Certain permissions have been granted to qualified scholars, and so the argument is that they are authorized to collect it on behalf of the Imams.

This narrations, the argument goes, is about those who the Imams did not authorize.