If you want to change this line programatically, then you need to jump through a few more hoops as you cannot use the usual BAPIs to add or edit one of these lines. Below is the code to add a debit memo to the repair line;
"Open the sales order for manipulation
call function 'SD_SALES_DOCUMENT_READ'
exporting
document_number = ls_vbak-vbeln
status_buffer_refresh = 'X'
requisition_buffer_refresh = 'X'.
"Update our line item
refresh lt_itemkom.
clear lt_itemkom.
lt_itemkom-posnr = '900003'.
lt_itemkom-updkz = 'I'.
lt_itemkom-vkgru = '109'. "Debit memo
lt_itemkom-menge = ls_vbap-kwmeng.
lt_itemkom-matnr_g = ls_vbap-matnr.
lt_itemkom-arktx_g = ls_vbap-arktx.
append lt_itemkom.
perform repairitems_generate(sapfv45p) tables lt_itemkom
lt_serikom
using ls_vbap-uepos.
"Save the order again
call function 'SD_SALES_DOCUMENT_SAVE'
exporting
synchron = 'X'
status_buffer_refresh = 'X'
requisition_buffer_refresh = 'X'.
commit work and wait.