Invoice factoring — no advance on unverified paper, no double-selling

invoice_factoring.cell

// ============================================================================
// INVOICE FACTORING — no advance on unverified paper, no double-selling
// ============================================================================
//
// Greensill Capital collapsed in 2021 holding billions in "invoices" that
// were never verified — some for trade that hadn't happened, some financed
// twice through different vehicles. The two frauds are structural here:
// an invoice can only be SOLD from the verified state (the issued -> sold
// edge does not exist), and sold has no edge back to itself or to verified,
// so the same invoice cannot be advanced twice. The advance ledger caps
// the cash at 90% of face value and refuses negative writes.
// ============================================================================

cell Factoring {
    face {
        signal issue_invoice(id: String, face_value: Int) -> Map
        signal verify_delivery(id: String) -> Map
        signal sell_invoice(id: String, advance: Int) -> Map
        signal collect(id: String) -> Map
        signal write_off(id: String) -> Map
        signal advance_paid(id: String) -> Int
    }

    memory {
        advanced: Map<String, Int> [persistent, consistent]
        invariant advanced >= 0
        face_val: Map<String, Int> [persistent, consistent]
        invariant face_val >= 0
    }

    // collected and defaulted are terminal. The fraud edges are ABSENT:
    // no issued -> sold (unverified paper), no sold -> sold (double-sale).
    state invoice {
        initial: drafted
        drafted -> issued
        issued -> verified
        verified -> sold
        sold -> collected
        sold -> defaulted
    }

    on issue_invoice(id: String, face_value: Int) {
        if face_value <= 0 { return map("ok", false, "reason", "bad face value") }
        transition(id, "issued")
        face_val.set(id, face_value)
        advanced.set(id, 0)
        return map("ok", true, "state", get_status(id))
    }

    on verify_delivery(id: String) {
        transition(id, "verified")    // raises unless invoice was issued
        return map("ok", true, "state", get_status(id))
    }

    on sell_invoice(id: String, advance: Int) {
        let fv = face_val.get(id) ?? 0
        if advance <= 0 || advance * 10 > fv * 9 {
            return map("ok", false, "reason", "advance must be in (0, 90% of {fv}]")
        }
        transition(id, "sold")        // raises unless verified — and only once
        advanced.set(id, advance)
        return map("ok", true, "advance", advance, "face", fv)
    }

    on collect(id: String) {
        transition(id, "collected")
        let fv = face_val.get(id) ?? 0
        let adv = advanced.get(id) ?? 0
        return map("ok", true, "rebate_to_seller", fv - adv)
    }

    on write_off(id: String) {
        transition(id, "defaulted")
        return map("ok", true, "loss", advanced.get(id) ?? 0)
    }

    on advance_paid(id: String) {
        return advanced.get(id) ?? 0
    }
}

cell test FactoringTests {
    rules {
        assert issue_invoice("inv1", 10000).ok == true
        // GREENSILL MOVE 1: advance cash on unverified paper — edge absent
        assert_fails transition("inv1", "sold")
        assert verify_delivery("inv1").ok == true
        // advance capped at 90% of face
        assert sell_invoice("inv1", 9500).ok == false
        assert sell_invoice("inv1", 9000).ok == true
        assert advance_paid("inv1") == 9000
        // GREENSILL MOVE 2: sell the same invoice again — no sold -> sold
        assert_fails transition("inv1", "sold")
        assert_fails transition("inv1", "verified")
        // the ledger refuses a negative advance outright
        assert_fails advanced.set("inv1", -1)
        // collection is terminal; a collected invoice cannot default too
        assert collect("inv1").rebate_to_seller == 1000
        assert_fails transition("inv1", "defaulted")
        // default path on a second invoice, equally final
        assert issue_invoice("inv2", 5000).ok == true
        assert verify_delivery("inv2").ok == true
        assert sell_invoice("inv2", 4000).ok == true
        assert write_off("inv2").loss == 4000
        assert_fails transition("inv2", "collected")
    }
}