Customizing Vendor Item Posting Keys 

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       1.      In Customizing for Financial Accounting (FI), choose Financial Accounting Global Settings ® Document ® Line Item ® Controls ® Define Posting Keys.

       2.      Double-click the posting key that you post vendor items with (in the standard system, 31).

       3.      Select Sales-Related, if it is not already selected.

       4.      Choose Field Status and double-click Taxes.

       5.      Set EC Tax Data to Opt. Entry (optional entry).

       6.      Save the posting key.