FLB1 - Sending to Suspense Account

I have an amount that is sitting in out SAP lockbox and that belongs to a customer. However the customer still needs to respond back on which invoice should this amount be applied to.

In the mean time, I have to clear this invoice and credit it to our suspense account. Now after entering suspense account, when I click enter, the customer payment should reflect in the not assigned box, after which I need click on Difference postings where this amount can then show as a credit to the suspense account when I simulate the document.

However, the not assigned amount box is not showing any amount, and the customer payment is reflecting in the assigned box. As a result, when I simulate , the debit is showing the lock account, but I am not able to show an equal credit in the suspense account.

I'd like to know how do I get to credit the suspense account and clear the lockbox account currently showing a credit of the customer payment. Is there another transaction code that can be used.

Thanks.