Startup Idea : App for tracking who owes who money

My friends and I are always forgetting if we have paid each other back and who owes who what amount for concerts, dinners, etc. Was thinking of an app that you can select from your contacts / facebook friends, who owe you money or that you owe money too. This will include what amount is owing and a reminder of when to pay by. Sometimes it can be awkward asking particular people for money so the app can send reminders to pay if haven't done so by the due date. You can then pay through the app using something like Paypal api.