Find the answer to your question
Can I unacknowledge and order?
Is there a way to unacknowledge an order?
The quick answer is NO. If you are using LMS, and retrieving the SoldReport. You know you are to acknowledge orders (using OrderAck) so future SoldReports will only contain unacknowledged orders or line items. Once you ack those orders there is no way to reverse/cancel the request after it is completed. So there is no way to unack the order. If you still need to gather information for those orders, I would suggest using the GetOrders API to retrieve the necessary information.
GetOrders - http://developer.ebay.com/DevZone/XML/docs/Reference/eBay/GetOrders.html
SoldReport - http://developer.ebay.com/DevZone/merchant-data/CallRef/SoldReport.html
OrderAck - http://developer.ebay.com/DevZone/merchant-data/CallRef/OrderAck.html