Hi Monchil ,
I have a similar requirement in which i have Update BKPF-XBLNR & INFVO-XBLNR during MRRL with EXTERNAL ID field in VL31N . But i am putting a break point in function module POST_DOCUMENT and it is not stopping there . What should be done . It is a priority Requirement . Plz help . I am attaching screenshots . 
Thanks and Rgds ,
Dave