It's impossible for me to edit duration fields such as the Budget. These appear by default as "00:00:00", with the ability to move between the hour, minute, and second fields with the Tab key. However, overtyping in these fields either has no effect, or else the digits typed end up overwriting the colons, which cannot be reinserted. It's hard, if not impossible, to enter a valid time duration this way, so pressing enter after such an attempt usually just blanks the field. Attached is an error log.
The budget field should be editable, and in my testing it is, but the visual is buggy. The colons disappear, it can act funny, but with some playing around, you can indeed enter the budget.
Definitely a bug that should be fixed.
In case there's any confusion, the other fields of Time Spent and Budget Left are not editable. Those are based on the efforts which you can track or enter and edit by hand. The total effort time on an issue is used to calculate those budget fields.
Need that budget field too!
I managed to enter a budget!!
Spoiler: click with your mouse after the first zero, so you see 0|:00:00, and then type your hours.