You can add it on the Document or Line levels.

Elements

Document level

Use the AllowanceCharge element to specify an allowance on the Document level:

Element	Cardinality	Description
Allowance/Charge Group (`Invoice/AllowanceCharge`)	0..n	Groups information about Allowances and Charges.
Charge Indicator (`Invoice/AllowanceCharge/ChargeIndicator`)	1..1	True or false flag that indicates if this element is modeling an allowance or a charge. If set to false, it is an allowance. If set to true, it is a charge.
Allowance/Charge Reason Code (`Invoice/AllowanceCharge/AllowanceChargeReasonCode`)	0..1	Code that specifies the reason for the allowance or charge. For countries without specific required values; you must use the UNCL 5819 codes listed here for allowances and the UNCL 7161 codes listed here for charges.
Allowance/Charge Reason (`Invoice/AllowanceCharge/AllowanceChargeReason`)	0..1	Text that describes the reason the Allowance or Charge is permitted.
Allowance/Charge Percent (`Invoice/AllowanceCharge/MultiplierFactorNumeric`)	0..1	The percentage that may be used, in conjunction with the document-level allowance base amount, to calculate the document-level allowance or charge amount.
Allowance/Charge Amount (`Invoice/AllowanceCharge/Amount`)	1..1	The amount of the charge or allowance.
Allowance/Charge Base Amount (`Invoice/AllowanceCharge/BaseAmount`)	0..1	The base amount to which a charge is added or an allowance is subtracted.
Allowance/Charge Tax Category (`Invoice/AllowanceCharge/TaxCategory`)	0..n	A group of elements providing information about the tax category of the allowance or charge.
Allowance/Charge Tax Category Code (`Invoice/AllowanceCharge/TaxCategory/ID`)	1..1	The codes that represents the Tax Category for the allowance or charge.
Allowance/Charge Tax Rate (`Invoice/AllowanceCharge/TaxCategory`)	0..1	The percent tax rate that applies to the allowance or charge.
Allowance/Charge Tax Exemption Reason Code (`Invoice/AllowanceCharge/TaxCategory`)	0..1	A code that represents the reason for an exemption for the allowance or charge.
Allowance/Charge Tax Exemption Reason Text (`Invoice/AllowanceCharge/TaxCategory`)	0..1	A text that describes for an exemption for the allowance or charge.
Allowance/Charge Tax Scheme Group (`Invoice/AllowanceCharge/TaxCategory`)	1..1	Groups information about the Tax Scheme that applies to the charge or allowance.
Allowance/Charge Tax Scheme Code (`Invoice/AllowanceCharge/TaxCategory`)	1..1	The code that represents the Tax Scheme.

Line level

Use the InvoiceLine/AllowanceCharge element to specify an allowance at the line level:

Element	Cardinality	Description
Invoice Line Allowances and Charges (`Invoice/InvoiceLine/AllowanceCharge`)	0..n	Groups information about Allowances and Charges.
Invoice Line Charge Indicator (`Invoice/InvoiceLine/AllowanceCharge/ChargeIndicator`)	1..1	True or false flag that indicates if this element is modeling an allowance or a charge. If set to false, it is an allowance. If set to true, it is a charge.
Invoice Line Allowance Charge Reason Code (`Invoice/InvoiceLine/AllowanceCharge/AllowanceChargeReasonCode`)	0..1	Code that specifies the reason for the allowance or charge. For countries without specific required values, you must use the UNCL 5819 codes listed here for allowances and the UNCL 7161 codes listed here.
Invoice Line Allowance Charge Reason (`InvoiceLine/AllowanceCharge/AllowanceChargeReason`)	0..1	Text that describes the reason the Allowance or Charge is permitted.
Invoice Line Percentage (`Invoice/InvoiceLine/AllowanceCharge/MultiplierFactorNumeric`)	0..1	The percentage that may be used, in conjunction with the document-level allowance base amount, to calculate the document-level allowance or charge amount.
Invoice Line Amount (`Invoice/InvoiceLine/AllowanceCharge/Amount`)	1..1	The amount of the charge or allowance.
Invoice Line Base Amount (`Invoice/InvoiceLine/AllowanceCharge/BaseAmount`)	0..1	The base amount to which a charge is added or an allowance is subtracted.

Restrictions

You must use the UNCL 5819 codes listed here.
For Allowances, you must set the ChargeIndicator to false. If you do not, the AllowanceCharge element is treated as a charge. See Charges.
For Document level Allowances, you must specify the sum of all allowances in the AllowanceTotalAmount subelement of the LegalMonetaryTotal element.
For Document level Charges, you must specify the sum of all charges in the ChargeTotalAmount subelement of the LegalMonetaryTotal element.

Country-Specific configurations

The following country configurations exist:

Examples

Document level allowance

The following is an example of a discount:

<Invoice>
    <!-- code omitted for clarity -->
    <cac:AllowanceCharge>
        <cbc:ChargeIndicator>false</cbc:ChargeIndicator> 
        <cbc:AllowanceChargeReasonCode>65</cbc:AllowanceChargeReasonCode>
        <cbc:AllowanceChargeReason>Production error discount</cbc:AllowanceChargeReason>
        <cbc:Amount currencyID="EUR">300</cbc:Amount>
        <cac:TaxCategory>
            <cbc:ID>S</cbc:ID>
            <cbc:Percent>25</cbc:Percent>
            <cac:TaxScheme>
                <cbc:ID>VAT</cbc:ID>
            </cac:TaxScheme>
        </cac:TaxCategory>
        </cac:AllowanceCharge>
    <!-- code omitted for clarity -->
</Invoice>