Here is the list of possibilities in patient payment posting process and what is the correct method to post.

1. Payment is equal to Balance.
Post the entire payment according to balance.

2. Payment is lesser than balance.
a) Check any notes in the statement as which DOS to post.
b) If no notes in the statement, see the statement history if payment match with statement then post as per statement.
c) or else post from oldest DOS.

3. Payment greater than balance.

a) Check whether its a duplicate payment.
b) Post the balance and keep remaining in open credit.

Patient payment at time of visit.

1) Generally patient pays copay for current DOS hence if there is no notes in the payment then post into the current DOS.
2. If patient pays more than copay, it may be some for today visit and remaining for old visit. Hence post accordingly.