How Purchase Discount works in vendor Bill?

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Hello, In 0.92 odoo site, when create vendor bill from purchase order, we want to set purchase discount.

And then we select Discount head (800101 Discounts) in the “Discount Account Head” like bellow. And when try to validate, get Validation error “Wrong credit or debit value in accounting entry! Credit or debit should be zero.”
We are halt in this case. Please help me how can use discount in purchase product/Vendor bill. And what will be the Discount head? Thanks in advance.