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.