The following WIKI Page will explain how to customize the filed BSEG-ZUONR as modifiable.
Depending on the customizing, the Assignment field might not be modifiable in some cases.
If the assignment field is defined in any table of FI-SL, this will not be possible to be changed. This functionality is explained in detailed in the notes 79760 and 827413.Review your customizing in Transaction OB32:
With the above settings you are customizing the field ZUONR as a modifiable field for Customer line items posted with a special G/Lfor down payments.
Please note that in addition to the document change rule, there are still a few situation where you can not change BSEG-ZUONR.
Also please check SAP Note 47245 for special ledger related matters.
In addition, the field allocation is not modifiable in case the line item is reversed.
If you are not allowed to change the field ZUONR because you move them to a special ledger (in an FI-SL variable field movement, please check with transaction GCF2). As of release 3.0*, there is a check to block such BSEG fields from changing with FB02 (despite any table entry) in order to prevent FI and FI-SL going out of synch - if we did allow you to change these fields, the changes would not pass over to FI-SL.
Check if any substitution is active which causes the entry in BSEG-ZUONR. The substitutions are defined within transaction OBBH.
You should be aware that BSEG fields which are used as sender fields in field-movements in SL are not foreseen to be changed in the original documents (in FI). The reason for this is, that changes would not be transferred to SL and so data inconsistencies between SL and FI would be the consequence.
The SAP Note 79760 describes a workaround. Please be aware of the consequences on following the steps stated in this note.
SAP Note 47245: BSEG-ZUONR not changable in spite of TBAER entry.
SAP Note 79760: Certain BSEG fields cannot be changed (FB02).
SAP Note 827413: FI: Changeability of fields in FI documents.