Since the invoice number is to be updated into the z-table after the invoice is posted, you cannot use a USEREXIT.
You have to use a BTE (Business Transaction Event). You need an experienced ABAPer to code this.
The transaction code to access BTE is FIBF.
Under the list of BTEs available, the following BTE would suit your prupose:
00001050 POST DOCUMENT: Accounting interface
The description for this BTE reads as:
Functionality
This function module is an interface description only and does not
contain any instructions. It is intended for developers of those
additional components that provide their own function module at Event
00001050 (Update posting, FI interface).
Function module SAMPLE_INTERFACE_00001050 must not be changed.
If you want to create your own function module that is to be accessed at
the callup point mentioned above, proceed as follows:
1. Copy the function module to your own name range.
2. Do not change any transfer parameters !!
3. Write your own coding in the copied function module.
4. Save and activate your function module.
5. Carry out the necessary settings in Customizing. Business
Transaction Events
EVENT
This Event is reached after calling up the update function module for FI
(POST_DOCUMENT) in the form routine FI_DOCUMENT_POST. One of the
function modules in which this routine is called up is FI_DOCUMENT_POST
(FI interface).
INTERFACE
All data relevant to the current document is transferred. The indicator
I_XVBUP serves soley to inform you that the SAP standard system posts
asynchronously.
It follows that no database activities involving the writing of data may
be carried out in the additional component.
The update including that of the tables specific to Add-ons are carried
out by the COMMIT WORK of the standard process. If your function module
is not accessed via RFC, you should always initiate asynchronous update
posting using the ABAP statement
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