Tax code not editable when invoicing outbound delivery invoice request

[SAPSupport]

Dear SAP support team,
we would like to change a tax code on invoice created from outbound delivery invoice request. When click on "invoice" field from the invoicing request screen view we go to item level and would like to change the tax code on first line. In the Taxes tab, the tax code field is not editable. How can we change the tax code? Why is it not editable anymore? 
Thanks in advance

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[SAPSupport · Accepted solution]

When there is a down payment assigned, the tax information is not editable in new invoice.

This is the current system behavior as designed. If you would like to change the tax information in new invoice, you may invoice the invoice request without down payment assignment.

reference KBAs:  

• 2408717 - When Invoicing an Invoice Request, Tax Code is Not Editable if Down Payment is Assigned 
• 2809206 - How To Work With Down Payments in Sales