In the category pop up when you click on a category, there should be a way to move up to the category's remaining amount to another category.
What it should look like:
A text box labelled "Amount:" with its text pre filled to be the category's remaining amount.
So if Groceries has 100 remaining, this text box should show 100.
A text box labelled "To:" with a select box listing all the categories, titled by their respective category groups. To the right of each category, there should be the category's remaining amount.
A "Move" button..
What should happen:
When the user presses Move, the category budgeted amount should be reduced by the amount the user wants moved. When that runs out, then the remaining amount should be correspondingly reduced by the difference.
Then the amount the user wants moved should be added to the chosen category.