If you want the payment to be included in the same column as what is
owed, then you will have to enter it as a negative amount.
Alternatively, you could have one column for what is owed (as you have
now) and another column for payments, and each column could have its
own SUM( ) formula, the difference of which at any point in time is the
amount outstanding.

As your list of transactions will grow ever longer, I would suggest
that you put the SUM( ) formulae on the top row (maybe with a different
background colour to stand out) and to freeze the panes so that the row
is always visible even if you are entering data on row 200 (say). The
SUM formulae could have a very big range, eg C2:C65522, so you don't
have to worry about adjusting it as you add new transactions.

I would suggest that you have a date column in A and brief descriptions
in column B.

Hope this helps.

Pete