You must remove the RETURNING phrase from the signature; instead use property-value for the return value in the indexer's getter method. For example:
indexer-id string.
procedure division using by value idx as binary-long,
returning return-str as string. *> remove this phrase
getter.
set return-str to "return-value" *> and this line
set property-value to "property-value"
goback
end indexer.