Trying to do it from the Stake level and Headquarters is only an option when selecting "Ward Missionary" as the From category. Any other one I select which has our excess funds (all Budget categories) causes the Headquarters option to disappear from the To Category and requires a unit number for external transfer. I can leave the "From" Category blank, but the system does not allow the transaction to complete. All of my From Categories are Budget categories (except for the Ward Missionary one).
For "From" you must use "Budget: Budget Allocations"
For "To" select "Out of Unit" then the top entry in the pull-down will be "Headquarters"
The form seems a little finicky, so if it doesn't work the first time, start over and try again.