The FOR UPDATE clause is intended for locking specific data (which is avaialble for reading from a transaction belonging to another connection) in advance while it is being read, to avoid deadlocks later, when it will be written. We do not recommended that you use the FOR UPDATE clause as it is irrelevant in managed lock mode.
SELECT
Doc.Ref
FROM
Document.RetailSale Doc
WHERE
Doc.Ref = &DocumentRef
FOR UPDATE AccumulationRegister.MutualSettlementsByAgreement.Balance
SELECT
Doc.Ref
FROM
Document.RetailSale Doc
WHERE
Doc.Ref = &DocumentRef