Withholding tax on payment minimum amount at invoice level and prepayment process

[Nicolas3]

Hello,

I have a requirement to manage Withholding tax on payment with a minimum base amount of 1000 TND at invoice level, managing withholding tax on prepayment.
Let say the WHT is 1%, with the following customizing:

For instance :
1/ Post Invoice

Vendor : Cr 1000 TND

2/ Prepayment request (F-59):

Vendor : Cr 500 TND

3/ Prepayment request payment (F-53)

Vendor : D 500 TND
Bank : Cr 495 TND
WHT : Cr 5 TND

3/ Payment of the balance (F-53)

Vendor Clear invoice : D 1000 TND
WHT : Cr 10 tnd
Vendor Clear prepayment : C 500 TND
Vendor Clear prepayment WHT : D 5 TND
Bank : Cr 495 TND

The issue is that SAP is not calculating WHT on the prepayment when it is under the threshold of 1000 TND, even though the linked invoice is above.
If both the prepayment and the invoice are above the threshold, this works properly (WHT is calculated during prepayment, and then during the final payment it is reversed and the total WHT amount is calculated on the invoice).
I tried the following :

1/ Change WHT Type by selecting Minimum Check : "Min base amnt Check at doc.invoice level"

2/ Change WHT Type by selection Central Invoice : "Central Inv. Prop" + Minimum Check : "Min base amnt Check at doc.invoice level"

3/ Forcing the WHT Amount on the prepayment request: it works during the payment of the prepayment request, but later on when paying the balance SAP does not calculated WHT on the invoice
4/ For some reason, while i ticked "Manual W/Tax amnt" on the WHT type, I cannot change the amount of prepayment request, and I cannot change the amount at Invoice payment if I forced it in the prepayment request

Thanks a lot for your help and insight I am stuck!

BR