but the amount paid has changed

Not ideal, but if there are not a lot of invoices, you could just follow the related records from PO to Invoice to Payment applied to invoice for each..

There are like 100 invoices and I still don't know where to see whether the amount applied to a particular invoice changed

How about a Search: Create a Transaction Search Criteria: Type = is Vendor Payment System Notes: Date (whatever day/range you think change was made) System Notes: Type = "is change' System Notes: Field = "is amount" (or some variation/ mine says 'amount in usd' You could probably add another criteria for who the vendor is if known Then add whatever results you need - but would include System Notes Field 'Old value' and System Notes field 'New Value'

? this is a customer payment

Ah sorry same idea though / perhaps even a little easier - so you have the customer,

there's no change in document status. It's going from Open to Open, the amount due is changing but that doesn't seem to show in the system notes