yeah you are right.
F-28 doesn't give that capability and even after trying through other incoming payment transactions with spl gl indicators, system gives hard error.
How ever, this scenario shouldn't be unique to my client alone. It is very much possible to receive partial payments from with out prior notice and its more time consuming task to create additional payment request for the balance of unpaid money (corresponding to initial payment request).
If any one has this scenario and provide your thoughts, your help much appreciated.
Thanks.