issue with billing date and copying tax condition

Hello,

We have successfully completed our implementation project. Thanks everyone for your support.

Currently, my client wants to create backlog invoices so they have to back date their billing dates so that the Posting dates will be backdated too because the Billing date determines the posting date. The Problem with this backdating is that the Tax condition is not copied or determined when the Sales Order is copied into the Invoice and it is a required field. therefore generate a pricing error "Pricing error: Mandatory condition MWAS is missing Message no. V1801"

I need the Tax Condition to be determined even though the Billing dates are backdated.

Your guidance will be appreciated,

Regards,

Patrick