The following is my understanding from your requirement:
You will pay the ST amount to vendor.
i.e. Total Bill value is Rs. 1145
Vendor : Cr : 1145
Services Ac : DR : 1000
ST : DR : 145
Maintain the condition type "JSVD IN:Service tax debit" for the Tax code in FV11 or FTXP (as per your Taxation Procedure) and check the entry with this Tax Code. (Create a new tax code if the existing tax code is already used).
Gopal.