Doc date blank in Ml18N screen

Hi

We want to capture the "service rendered date" in the place of doc date in Ml81N screen and make it mandatory, by defualt doc date has today's date. How can we make the date Null in this field.

These are the BADI's i found out for Ml81N t-code but our ABAPer said he cannot make change in anyone of them .

MB_QUAN_CHECK_BADI

MB_STOR_LOC_BADI_GTS

BADI_PRKNG_NO_UPDATE

BADI_MATN1

AC_QUANTITY_GET

MB_DOCUMENT_BADI_SAP

MB_DOC_BADI_INTERNAL

Please suggest how can i proceed.

Thanks,

Vaishnavi