If you have already paid your share to someone, you just need to delete the corresponding row. Swiping the row shows two options:
Pay off: meaning they are settled and the debt is paid in full
Amount: meaning just a give amount is paid
Note: Depending who owes who, the amount goes to the person who has to get paid.
Sam owes John $41.33.
If you swipe the row and tap Pay off, that means Sam paid John $41.33
If you tap amount, it asks for the amount. This means Sam paid John the amount you entered.