Change Ledger::qty to a number field. Select field options and specify Looked up value on the auto-enter tab. Specify Journal::qty as the value to be looked up into the field.
Dismiss Manage | Database and go to a Ledger layout that has the _kf_trans_id field on it. Select show all records, put your cursor in this field and choose "relookup" from the records menu. This will copy the data from Journal::qty to Ledger::qty.
Once done, you can delete your qty field from journal and remove the looked up value setting from ledger::qty.
Done. However, after selecting "Relookup Field Contents" from the Records menu while my cursor is in the Ledger::qty field, FM gives me the following dialogue: "There are no fields that look up values based on the field "qty".
Update: In Ledger layout, when I put my cursor in the Ledger::_kf_trans_id field instead of the Ledger::qty field, and chose "relookup" again, it worked!