Calculate expenses based on category code

Not sure how to clearly explain the desired outcome, I think it's the reason why I can't think of what solution to implement.

• I would like the table on the far right to automatically calculate the expenses based on GL Code.
• I've created a drop-down menu for that column in the main area.
• The 'Allocated Budget' is manual input in far right table.
• The 'Expenses' is what I would like to be calculated based on the 'Total Spent' column (both orange highlighted).