Expenditure and transfer — business logic

Expenditure writes goods off stock without a sale to a customer: a transfer
between the company's own warehouses, a shipment with no revenue, a
write-off, a return to a supplier. It differs from selling in exactly this
respect — money isn't recorded as revenue on an expenditure document (see
Selling for the opposite case; both share the same
processing).

Goods are taken only from real stock

An expenditure line isn't a free-form position — it's a "slice" of a
specific warehouse lot at the moment it's picked: the item, quantity, and
lot price are fixed into the document line. You can't write off more than
is in stock — this check is mandatory both when the line is picked and
again during processing. If the lot changed between picking and processing
(someone else processed another document in between) — processing is
rejected, not carried out against stale data.

If a single lot doesn't hold enough of an item, expenditure is
automatically split across several lots on a "oldest first" basis, with
user confirmation.

What happens on processing

For each line:

1. Source lot check — the item and price must match what's fixed in the
   line (otherwise processing is rejected).
2. Quantity check — the lot must have enough.
3. Write-off from the lot: quantity is reduced; if the remainder reaches
   zero, the lot is marked inactive but not deleted (lots are never
   deleted, only exhausted — that's what makes up the history).
4. Reservation release — if there was a soft reservation on this item for
   an order or document, the reservation is reduced by the shipped
   quantity (never more than existed).
5. If a destination warehouse is specified — this is a transfer: a new lot
   is created (or topped up) at the destination. The cost basis at the
   destination becomes equal to the shipping price of the source line —
   meaning a transfer can "raise" or "lower" the item's booked cost at the
   new warehouse. The receiving side of the transfer is created
   automatically, already in a processed state — the user doesn't enter it
   separately.

Why the price can change on a transfer: cost basis isn't an abstract
number, it's a property of a specific lot. The lot physically moves to
another warehouse at whatever price it was shipped at — if that price
differs from the original (say, it includes a share of shipping costs), the
lot legitimately costs more or less at the new warehouse.

Reservation isn't always released

Reservation release on expenditure only happens if a partner is specified
("shipment on credit" to a specific counterparty) — an ordinary transfer
between the company's own warehouses doesn't touch reservations, because it
isn't "promised" to anyone. For selling, the reservation is always released
(see Selling).

Debt or payment

Same logic as receiving, just in the opposite direction: if a partner is
specified but no account is, the document amount is subtracted from their
balance, creating a debt ("shipped on credit"). If an account is specified,
a cash document twin is automatically created on processing for the
expenditure amount — money is considered received right away, the
partner's balance isn't adjusted.

Reporting: transfers are counted separately

Ordinary receiving/expenditure registers deliberately exclude transfers —
the goods never left the company, so for a "how much left for good" report
that isn't an event. Transfers have their own register instead, one
warehouse at a time, with "gave" and "received" columns (a single warehouse
can be both source and destination across different documents in a period).