No due items exist at the key dat Message no. F5345

Hi Gurus

When I try to reverse this item BOE (because payment is received) through the T-Code F-20 ,SAP gives me the below error message (u201CNo due items exist at the key dateu201D Message no. F5345) I have checked the customer line item and document is due for 31.01.20111 and Due by date: 04.03.2011(entered F-20). So what could be th ereason fo rthis error?

Thnaks

Sanju