When we upgrad to 4.6x, users was not willing to access the FI document through Invoice display :-

MIR4 -> Follow on documents

I agreed with them because it is really troublesome to go through such a long steps in order to find out the FI document number.

In 4.6x, SAP only display the Invoice document number after posting.

In order to display both the Invoice and FI document number at the same time, you have to apply OSS notes 216935 and 310231.

You need to changed the programs in order to get the message 
"Invoice Document no. & and FI Document no. &  created."

The following messages which were modified  as per the OSS note to get the FI document number.

Message no
Newtext                                    Old text
 060                                           Document no. & created
 Invoice Document no. & and FI Document no. &  created

 075                                           Document no. & created (blocked for payment)
 Invoice Document no. & and FI Document no.& created (blocked for payment) 

 282                                            Document reversed with no. &: Please manually clear FI documents (No change done in 
                                                  the message text).  But it will display FI document number after changes.

 392                                            Invoice document & has been posted (Now this displays Invoice document number. 
                                                   After changes,it will display FI  document number).

After these changes you  will be able to see the FI document number and Invoice document number displayed in the message bar after executing the MIRO Transaction.

You will need authorizations and access key to do the modification.

See Also
FI Errors and Probable Solutions

SAP Books
SAP FICO Books  - Certification, Interview Questions and Configuration

SAP FICO Tips
SAP FI/CO Tips Financial Accounting/Controlling

Main Index
SAP ERP Modules, Basis, ABAP and Other IMG Stuff