Hi Rama,
I believe this WHT is required for India. If that is the case then as per Indian rules for WHT, i understand that WHT needs to be deducted for Invoice and payment whichever happens earlier.
SAP has provided standard solution for that. Please make use of Business place and section configuration.
Define WHT at both Invoice and payment level.
Once that is done, system would deduct WHT at both levels but would reverse one whichever is happening later.
Like suppose you post the DP first, system would deduct WHT, now when you post invoice, system would again deduct WHT but when you are going to clear this, system would reverse the WHT posted during invoice.
Hope this is clear.
Thanks and Regards,
Gaurav