After googling this topic, I have no idea why they're so few results related to Temporary Storage of FSCM Dispute Management(FDM). May because it's rarely been used or has this functionality been abandoned at S/4 HANA?
Here is a list of basic knowledge about this for reference:
1. Temporary Storage Table: FDM_BUFFER
The key field 'RELID' will be fixed as 'DM' for dispute cases;
The key field 'CASE_GUID' is the same as UDMCASEATTR00-CASE_GUID;
The fields 'NEW_SEQNR' and 'OLD_SEQNR' are the new/old Sequential Number of Processes that could be compared with UDMCASEATTR00-FIN_PROC_SEQNR.
2. FM related to FDM Temporary Storage
Create FM: FDM_BUFFER_INSERT
Delete FM: FDM_BUFFER_DELETE
Notice FM: FDM_BUFFER_NOTIFICATION
3. Program related to FDM Temporary Storage
Once got an SBWP message after performing FI clearing which has a dispute case against it:
Postprocessing for Dispute Cases and Promises to Pay Program: FDM_PROCESS_BUFFER
The main FM for this program is: 'BAPI_DISPUTE_PROCESS', 'UDM_DC_CHANGE'. The error message Process step &1: Change not possible, process step &2 missing UDM_MSG(037) comes from routine 'check_seqnr' of 'UDM_DC_CHANGE'.
The major check is:
Check whether UDMCASEATTR00-FIN_PROC_SEQNR and I_PROC_SEQNR_OLD(FDM_BUFFER-OLD_SEQNR) are the same. Otherwise, there is a pending process that has to be performed first.
It means whenever there's one version of Temporary Storage has been created, the latest version of the old Sequence number at FDM_BUFFER should match FIN_PROC_SEQNR for the current dispute case. Otherwise, check why there's un-processed Temporary Storage or no update of UDMCASEATTR00-FIN_PROC_SEQNR.
I find it's hard to trace how FDM_BUFFER has been filled through its entry has a timestamp. Please share if you are familiar with how FDM_BUFFER has been filled, thanks!