help with relationships or structure

[Neuronal Nerd]

I am trying to setup a solution to track patient's appointments and their payments for those appointments. Because appointments might need multiple payments and because paments need to sometimes be split across multiple appointments. I think I have a many to many design and need a join table. Attached is what I've set up so far, but it is not working as I would like. I would like to enter in the payemnt info and then choose which appointment to apply that amount too. However, that portal where I'm entering in payment is creating unwanted records.

Patients.zip

[Neuronal Nerd]

Just to further clarify my question: when I enter a new payment in to the appointments payment table, it automatically creates a new record with the next serial ID (for example: POS0002905 pops up); however, I would like to apply the payment to an existing appointment, now a new one). I have existing appointments in a value list. Even though I can just change to the new serial number to the one I want, it may not be obvious to the user that his or her entered serial number switches to the next sequential one when the record is committed.