Wallet infrastructure with a ledger you can audit line by line.

# Fund a wallet, then send money. Balances only move on the ledger.
curl -sS -X POST "$WALLETD_API/v1/users" \
  -H "Authorization: Bearer $WALLETD_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: usr-42-create" \
  -d '{"external_id":"user-42","kind":"consumer","display_name":"Ada Lovelace","handle":"ada"}'

curl -sS -X POST "$WALLETD_API/v1/transfers" \
  -H "Authorization: Bearer $WALLETD_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: xfer-ada-grace-001" \
  -d '{"from":"user-42","to":"@grace","amount":2500,"currency":"USD"}'

client := walletd.New(os.Getenv("WALLETD_API"), os.Getenv("WALLETD_API_KEY"))

var transfer walletd.Transfer
err := client.Do(ctx, http.MethodPost, "/v1/transfers", map[string]any{
    "from":     "user-42",
    "to":       "@grace",
    "amount":   2500, // minor units, $25.00, never a float
    "currency": "USD",
}, &transfer, "xfer-ada-grace-001") // idempotency key
if err != nil {
    return fmt.Errorf("transfer: %w", err)
}

client = WalletD()  # reads WALLETD_API / WALLETD_API_KEY

transfer = client.request(
    "POST",
    "/v1/transfers",
    {
        "from": "user-42",
        "to": "@grace",
        "amount": 2500,      # minor units, $25.00
        "currency": "USD",
    },
    idempotency_key="xfer-ada-grace-001",
)
print(transfer["status"])  # "settled"

const res = await fetch(`${API}/v1/transfers`, {
  method: "POST",
  headers: {
    Authorization: `Bearer ${API_KEY}`,
    "Content-Type": "application/json",
    "Idempotency-Key": "xfer-ada-grace-001",
  },
  body: JSON.stringify({
    from: "user-42",
    to: "@grace",
    amount: 2500, // minor units, $25.00
    currency: "USD",
  }),
});
const transfer = await res.json();

$transfer = $walletd->request('POST', '/v1/transfers', [
    'from'     => 'user-42',
    'to'       => '@grace',
    'amount'   => 2500,      // minor units, $25.00
    'currency' => 'USD',
], idempotencyKey: 'xfer-ada-grace-001');

echo $transfer['status']; // "settled"

var body = Map.of(
    "from", "user-42",
    "to", "@grace",
    "amount", 2500,      // minor units, $25.00
    "currency", "USD");

Transfer transfer = client.post(
    "/v1/transfers", body, "xfer-ada-grace-001", Transfer.class);