The ledger

Every approved movement writes a line to an append-only ledger: nothing is ever changed or deleted after the fact, only added. That's what guarantees a reliable history for an audit — accounting, funder, or internal.

Valuation at weighted average cost

Stock is valued at weighted average unit cost: every inbound movement recalculates the article's average price in the relevant warehouse, and every outflow (outbound, loss) is valued at that current average price rather than the original purchase price.

Lot-level traceability

An outbound movement or return can be precisely linked to the inbound line it came from (same article, same warehouse), with a check that the requested quantity doesn't exceed that lot's remaining balance. This lot-level traceability is independent from weighted-average-cost valuation: it tells you where an article came from, the weighted average cost tells you what it's worth.

Negative stock forbidden

An outbound movement or loss that would make the theoretical stock negative is rejected at approval time — the check happens within the transaction, as close as possible to the actual write to the ledger.