basic formula help needed

Hi guys I would be grateful if someone could advise me on the correct formula to use for the following task.

Please see the attached spreadsheet (excel 2007)

Column "K" is the cumulative profit for reference "KH". I want this column to total up all the totals in column J.

The cumulative profit (column K) is a sum of money that is owed to a partner in a joint business venture. I need column's L and M to reflect what money had been paid and what money is owed - is that possible?

For example, you can see that the cumulative profit for KH so far is 764.18. If I were to enter "500" in column L, I would want column M to show that 264.18 is still owed......

Hope that makes some kind of sense.....

thanks