There has to be at least one field common between the 2 'groupings' that can hold a value that ties groups of records together, but that field must hold different values. For example, if you're wanting to have a line item approach (like a sales order) where 1 entity can perform aggregate functions over a group (like recording multiple sources of income and summing them) the incomes must pertain to the same person and all the required fields have values, but they are differentiated from one another by IncomeID (for example). If you don't have this sort of setup, then I have to wonder if your db is properly normalized as a whole.
Not 100% certain, but your case could be one of the few where storing calculations is required. If not, it's because all income sources and all expenses should be separate records and the form performs the calculations based on the sums of your fields. That is to say, sum of incomes minus sums of expenses = remaining balance, in which case the amounts should not be calculated then stored at all. Also, if you have expenses and/or incomes going left to right in the fields of a table (like a spreadsheet) then your db is definitely not normalized. Continuing on with it will likely cause you more grief as you go.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.