Posting Illegal Statement Error

Hi..

Can anyone help me with my problem. It's like this. I have an output type ZRD3 in billing document (VF01) which performs special function (8). this output type calls my routine to perform automatic posting of credit memo request. I used call transaction (for BDC) and submit command but the error says:

POSTING_ILLEGAL_STATEMENT

This program is triggered in the update task. There, the following ABAP/4 statements are not allowed:

- CALL SCREEN

- CALL DIALOG

- CALL TRANSACTION

- SUBMIT

If I would delete the call transaction and submit command, my program would work but i need the 2 commands for my program to run correctly and perform its function. Any suggestions?

Thanks..