Hi Gerald,
Standard Navision will certainly allow to delete the Open Sales Invoice with the same Document No of Posted Sales Invoice, Please check for Sales Header Table -On Delete trigger for any customizations.
I dont have much details from your query . It is not recommended to have same No Series for Unposted and Posted Orders-Invoices (Navision will clearly capture the Document No to Posted table for link and future References)
Regards,
Anjan