2887991 - VBAP-FKREL is empty hence proforma invoice cannot be billed

The original issue in this case is proforma invoice cannot be billed via VF01 : The reason for that is because VBAP-FKREL is empty in sales order item, billing relevance is set to B for item category you use.
Then why VBAP-FKREL is empty?

This FKREL field gets empty by ARM ABAPcode:

CL_MSR_SALES================== / CL_MSR_SALES=
METHOD / SAVE_PREPARE (CL_MSR_SALES)

if is_vbak-vbtyp eq IF_SD_DOC_CATEGORY=>RETURNS and
( ls_comm_ctrl_trc-refund_control eq if_msr_trc_c=>gc_refund_after_cm or
ls_comm_ctrl_trc-refund_control eq if_msr_trc_c=>gc_refund_by_cmr or
ls_comm_ctrl_trc-refund_control eq if_msr_trc_c=>gc_refund_none or
ls_comm_ctrl_trc-refund_control eq if_msr_trc_c=>gc_refund_approved or " note 1696961
ls_comm_ctrl_trc-refund_control is initial or
ls_comm_ctrl_trc-refund_type eq if_msr_trc_c=>gc_refund_type_replacement and
ls_comm_ctrl_trc-refund_control eq if_msr_trc_c=>gc_refund_immediately ).
if ls_xvbap-fkrel is not initial.
clear ls_xvbap-fkrel.

Read more...

Advanced Return Management (LO-ARM)

SAP S/4HANA all versions ; SAP enhancement package 4 for SAP ERP 6.0 ; SAP enhancement package 5 for SAP ERP 6.0 ; SAP enhancement package 6 for SAP ERP 6.0 ; SAP enhancement package 7 for SAP ERP 6.0 ; SAP enhancement package 8 for SAP ERP 6.0 ; SAP enhancement package for SAP ERP, version for SAP HANA all versions

Release CMR for Material Inspection in Background Processing, , KBA , LO-ARM-SD , Sales , LO-ARM , Advanced Returns Management , Problem