Posting key in BAPI_ACC_DOCUMENT_POST

Hi All,

when i am using BAPI_ACC_DOCUMENT_POST.

it executes successfully but

if i enter the amount with + sign it uses 01 as the posting key.

if i entered the amount with - sign it selects 11 as posting key.

but requirement is instead of 11 I want 50 as posting key.

how it determines the key?

and can i change the posting key?

How can I pass the posting key field to this BAPI?

Thanks and regards,

Sri Lakshmi.