Paymnt diff while posting through cash desk

Hi Experts,

I want to post a paymnt diff while posting thru cash desk. However, while selecting invoices in cash desk, the system is not allowing the paymnt amount to be lower than the amount selected.

pls guide as to how the system can be configured to book exp automatically. i did define the PaytDiffExpAcct in posting area 0060

many thanks in advance!