Hello Sivakumar,
I understand your requirement to give Cash Discount only if Payment Term CD is used. But there is was system can copy Cast Discount amount from Payment term to a Z Condition Type without any linkage.
Also what is the business definition of Cash Discount in your case? Generally Cash discount is offered only if payment is made in mentioned number of days. Else entire invoice amount (without Cash discount) is payable. Thus Cash Discount in Invoice is only a provision and actual charge to expense and Customer account is done only when payment is done.
I request you to discuss with usage of Cash Discount with your FI colleagues and also go thorough SAP Standard documentation on Cash Discount.
Thanks,
Jignesh Mehta