Being able to add notes to the transactions was a great idea. What about expanding that to allow a notes field for the planned amount on the budget screen as well?
At the time of planning your monthly budget you would know how you got to a specific number but with time you may need a reminder on what transactions you were planning for that category in the month.
It happened to me this month where I set a specific value for the 'Home & Garden' category that must have made sense at the beginning of the month but now its not fully used up and I can't for the life of me recall what I missed. A notes field would go a long way towards helping here.