Get S/L partner while creating the posting document

Hello gurus,

I have one question regarding one issue I am trying to solve.

In the Tcode VF01, I am creating a billing document and it's generating some FI document.

On the FI document for one line item, I would like to get the SL partner linked to the account.

I have implemented and activated the user exit SDVFX004, on the structure xaccit there is all the information regarding the concerned line item and this SL partner field ( VBUND) is available.

I have modified the value field of xaccit-vbund but there is an issue, I get a message saying that the billing document has been created but the FI document could not be created, probably due to the modified field.

I would like to know if there is a way to get the S/L partner on the posting document while creating billing document.

Thank you for your help.

Best regards.