SAP SD Payment Terms

Question:
We raise invoice to a customer. Now if customer pays with in 21 days he gets 10 % disc and if he pays with in 30 days he gets 5% disc, How do we configure thisscenario?

Answer
In the above scenario, we can choose payment terms as one of the fields in the condition table: Payment terms can be defined as follows:

NT21 Within 21 days Due net (For NT21 the customer would get 10% discount)
NT30 Within 30 days Due net (For NT25 the customer would get 5% discount)

Upon selecting the relevant payment terms system would determine the percentage discount in the document.
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Now If the scenario is 'unless the customer pays the amount, payment date is not known hence we don't know which payment term to use and which discount to apply' as u mentioned. e.g.

e.g. payment terms => 21 days 10% cash discount; 30 days 5% cash discount
Then we have to use the condition type SKTO , it is a special condition type used strictly for this scenario i.e based on which payment term discount should be applicable. This condition type is not passed to accounting and generally not to COPA either (as you can see no Act keys for this condition type is not maintained in the Pricing Procedure)
The condition category E cash (in V/06) discount tells the system to go get the payment terms and calculate the potential/actual value i.e. 10% within 21 days and 5% within 30 days.

Based on the differing payment terms while payment, Invoice value will not change and would be the same, but SKTO will correct the value and discount is calculated in A/R instead.
The discount is applied on posting of the invoice and an error will be raised if the payment amount does not equal the net - the calculated cash discount. In the above case, if the payment is within 30 days, then system will throw an error if the customer takes 10 % instead of 5%.

Hope I am able to explain: Post your doubt/s via comment/s