Validation \ User-exit \ BADI \ BTE for F-43 for missing exchange rate

Question

Hi,
I maintain the exchange rate on a daily basis in TCURR exchange rate table.
But, let's say that, by accident, in one day I fail to input the exchange rate.
In this case, when I input a invoice in F-43, I'd like for the system to issue a warning (blocking) message saying that there is no exchange rate for that specific posting (translation) date.
I tryed to create a validation for F-43. But the conditions available in validation are just too simple and I couldn't achieve my goal.
However, I saw I can attach an user exit to the validation or an user exit directly in F-43.
I found in SMOD the following user-exits for F-43 (in package FBAS), but I don't know how to use them:
F050S001 FIDCMT, FIDCC1, FIDCC2: Edit user-defined IDoc segment
F050S002 FIDCC1: Change IDoc/do not send
F050S003 FIDCC2: Change IDoc/do not send
F050S004 FIDCMT, FIDCC1, FIDCC2: Change outbound IDoc/do not send
F050S005 FIDCMT, FIDCC1, FIDCC2 Inbound IDoc: Change FI document
F050S006 FI Outgoing IDoc: Reset Clearing in FI Document
F050S007 FIDCCH Outbound: Influence on IDoc for Document Change
F180A001 Balance Sheet Adjustment
FARC0002 Additional Checks for Archiving MM Vendor Master Data
RFAVIS01 Customer Exit for Changing Payment Advice Segment Text
RFEPOS00 Line item display: Checking of selection conditions
RFKORIEX Automatic correspondence
SAPLF051 Workflow for FI (pre-capture, release for payment)
Can you help me to attach an user-exit to a validation or an user exit directly in F-43 to do the specified validation?
Or maybe this can be done with a BADI \ BTE?

Answer 1

Thanks for the reply.
I succeded creating the validation with an user-exit. I made the following steps:
1) I created the validation in GGB0 (for document header), issuing a standard message (420 "Exchange rate not found") from message class (ZRSAU) - which is a copy of RSAU message class (in the beggining I put no pre-requisites, checks or rules, just the message).
2) In OB28 I attached the validation (with call point document header) and I made it active (without batch input).
3) I copied the RGGBR000 program (which is standard program with an example of validation) into ZRGGBR00 (it should be maximum 8 characters).
Then I changed the ZRGGBR00 program (form U100) comparing the translation date (from BKPF) with the date from table TCURR (and exchange rate type M, and documents currency other than local currency).
If the program doesn't find any exchange rate in TCURR in that date, it issues the error message (assigned in GGB0).
4) In GGB0, I put the U100 user exit as a check and rule
5)  In GCX2, I attached the ZRGGBR00 program to the GBLR application area (which is for user exits for validations / rules).
Now it's working fine!