BADI - ME_REQ_POSTED

Hello experts

i need to implement BADI - ME_REQ_POSTED to change some fields in purchase request when save,

But i have some problem to change structure IM_EBAN.

this is the code

  data:  l_IM_EBAN2 like line of IM_EBAN.

  loop at IM_EBAN into l_im_eban2.

    clear l_im_eban2-afnam.

    modify im_eban from l_im_eban2.

  endloop.

But i have msg:The field "IM_EBAN" cannot be changed. -          

How can i change this field?

Thanks in advice

B.