Thank you Irene S
What I don't get is the SAP behaviour of placing the status of 'Applied' for overpayments and posting the residual amount on customer account.
This is not what happening if it is an underpayment. If the underpayment is beyond the tolerance ($25 in my case) the status is 'on-account' and amount posted as partial payment. This is what I am expecting in case of overpayment (tolerance is $0).
In OBAY I have the 'Partial Payments' unchecked.
Would you have more information on how overpayment can behave the same way underpayment works ?