Cards inside your app

// your ledger decides: the card spends the user's balance in your app
app.post("/guara/authorize", express.raw({ type: "application/json" }), (req, res) => {
  if (!verify(req)) return res.status(401).end();

  const { data } = JSON.parse(req.body);
  const balance = ledger.available(data.external_user_id);

  if (data.amount_cents > balance) {
    return res.json({ decision: "DECLINE", reason: "INSUFFICIENT_BALANCE" });
  }
  ledger.hold(data.external_user_id, data.amount_cents);
  res.json({ decision: "APPROVE" });
});