Here's how I'd go about it:

1. Figure out which invoices (or parts of invoices) should have been charged to which company.

2. Set up a new customer for the new company.  Enter all of NewCo's invoices into that customer as new sales.  Adjust dates as needed.... do not date any invoice earlier than the first date of your current fiscal year.  Use a variation of the original invoice numbers, for example if the original invoice was #123, number the new one 123. (note ending period).  You may want to use a comment on the invoices where you've needed to change the date.

3. In the old customer record, enter negative invoices to offset all of the invoices you've re-entered in NewCo in step 2.  Again, the earliest date you use should be the start of your current fiscal year.  And again, use variations on the original invoice #, such as 123R (for reversal) to undo part or all of original invoice 123.

4. In the old customer record, record 'receipts' to match the old positive invoices to the new negative invoices - the total receipts will be zero (reflecting the fact that OldCo didn't actually pay for this), but the old invoices or portions of invoices that were incorrectly billed to OldCo will no longer show as outstanding.

At this point, you will have a customer record for NewCo with only NewCo invoices, and a customer record for OldCo with only OldCo invoices.  Unfortunately, some of the invoice dates will be wrong, and there's no way to fix these dates in Sage.

5.  Call up a customer transaction detail report for OldCo, showing all the invoices you want to list for the liquidator.  Export the list to Excel, and fix correct the dates as needed in the Excel file.  Then, send the Excel file to the liquidator.