Integrating mobile money (MeSomb) into a legaltech platform: an escrow that holds

Integrating a payment gateway is the rarest feature where a bug isn't an annoyance, it's money leaving the system. On TribuneJustice, a legaltech where clients pay lawyers before the service is delivered, the stakes were higher: we needed an escrowflow, not just a “charge the card” button.

This is what integrating mobile money in Cameroon (MeSomb, via MTN Mobile Money and Orange Money) looked like, and the engineering that stops a payment stack from leaking money.

Why payments are a legaltech's riskiest code

On a legal platform, a payment is not “buy a product”. It's: a client puts money in trust, a lawyer performs a service, the funds are released. That means the payment engine carries:

• An escrow lifecycle (held, released, refunded) that must never skip a state.
• Two parties (client and expert) plus a platform commission in a single transaction.
• The requirement to handle failures, retries, and double-notifications without ever crediting twice.

If a standard SaaS can tolerate “the payment went through twice, we'll refund later”, a legal escrow cannot. Let's look at how I built it.

One abstraction, three providers

The first decision: never hard-code a provider. Every payment touches a gateway through a single interface, so swapping or adding a PSP never touches business logic.

interface PaymentGatewayInterface
{
    public function getGatewayName(): string;
    public function initiatePayment(PaymentInitDTO $dto, string $internalReference): PaymentResultDTO;
    public function verifyWebhook(Request $request): bool;
    public function handleWebhook(Request $request): PaymentResultDTO;
    public function checkStatus(string $paymentReference): PaymentResultDTO;
}

A router picks the right implementation per context: MeSomb is the default aggregator for all mobile money in XAF (MTN MoMo, Orange Money), while CinetPay/Flutterwave handle cards and international payments.

if (in_array($method, [Transaction::METHOD_MTN_MOMO, Transaction::METHOD_ORANGE_MONEY], true)) {
    return $this->mesomb;   // mobile money (XAF) → MeSomb
}
return $this->cinetPay;     // cards & international → CinetPay / Flutterwave

The hard part: signing requests like the SDK does

MeSomb doesn't use a simple API key. Every request carries an Authorization header built from an HMAC-SHA1 signature over a canonical request. The official PHP SDK (hachther/mesomb-php) constructs it in a veryspecific way, and if your header insertion order doesn't match exactly, every request is rejected with a 401.

The subtle details that make or break it:

• Headers must be inserted in the exact order: content-type (for non-GET), host, x-mesomb-date, x-mesomb-nonce.
• The canonical request joins the path segments rawurlencoded and hashes the body with sha1.
• serialize_precision must be set to -1 so PHP's JSON serialization doesn't produce subtly different floats.

// The scope and string-to-sign, rebuilt line-by-line from the SDK
$scope = $date->format('Ymd').'/payment/mesomb_request';
$stringToSign = 'HMAC-SHA1'."\n".$timestamp."\n".$scope."\n".sha1($canonicalRequest);
$signature = hash_hmac('sha1', $stringToSign, $this->secretKey);

Two more real-world traps with Cameroonian phone numbers:

• Users type their number in a dozen formats (+237677123456, 00237 677..., 67712 34 56). I strip everything non-digit, drop the 237 prefix, and keep the 9 local digits.
• The operator is inferred from the prefix (69, 655, 656: Orange; otherwise MTN), because the API needs to know which wallet to debit.

Idempotence: the bug that double-charges

The signature of every payment engine is a race condition. Consider a confirmation path where a webhook and a manual status check both arrive for the same transaction, or where the PSP retries a notification. Without protection, two concurrent requests both pass the if ($tx->status !== 'completed') guard, and you generate two invoices and mark the service as paid twice.

The fix is pessimistic locking inside the transaction:

return DB::transaction(function () use ($transaction) {
    $lockedTx = Transaction::lockForUpdate()->findOrFail($transaction->id);

    if ($lockedTx->status === Transaction::STATUS_COMPLETED) {
        return $lockedTx; // already handled → idempotent
    }

    $invoice = Invoice::create([...]);
    $lockedTx->update(['status' => Transaction::STATUS_COMPLETED, ...]);
    // ... mark the ServiceRequest as paid
});

The same pattern guards every money-mutating transition: escrow release (early-return if already released) and refund (early-return if already refunded).

The escrow lifecycle

Every transaction starts pending with escrow_status = held. The funds are in trust, not with the expert.

'escrow_status' => Transaction::ESCROW_HELD,
'status'        => Transaction::STATUS_PENDING,

When the service is delivered, the escrow is released and the expert payout is scheduled at J+7 (an owner decision: give a cooling-off window). The platform commission (~20%) is computed up front and the net amount goes to the expert as a pending payout.

$scheduledAt = $now->copy()->addDays(self::PAYOUT_DELAY_DAYS); // 7

$lockedTx->update([
    'escrow_status' => Transaction::ESCROW_RELEASED,
    'payout_due_at' => $scheduledAt,
]);

ProfessionalPayout::updateOrCreate([...], [
    'commission_amount' => $commissionAmount,
    'net_amount'        => $netAmount,
    'status'            => 'pending',
    'scheduled_at'      => $scheduledAt,
]);

Each state transition is validated and immutable: you can't release funds from an already-refunded escrow, and the refund path is its own state machine.

Refunds as a state machine (ADR-001)

Refunds aren't one button. They're scenarios with different economics:

• Full refund (cancellation before start): 100% of the gross paid, PSP fees absorbed by the platform.
• Partial refund (cancellation after start): a percentage entered case by case by an admin.
• Dispute: 100% of the gross plus recovery of the platform commission from the expert, either debited from their internal balance or clawed back as a negative net payout (net_amount negative) absorbed on future payouts.

Webhooks: the part that breaks in production

Webhooks are where the money gets confirmed, and they're full of traps.

Signature verification with replay protection. MeSomb sends an X-MeSomb-Webhook-Signature header in the format t=<timestamp>,v1=<signature>. I verify it with HMAC-SHA256, reject anything outside a 10-minute timestamp window (anti-replay), and compare with hash_equals (constant-time, against timing attacks).

if (abs(time() - $timestamp) > 600) {
    return false; // replay attempt
}
$payloadToSign = "{$timestamp}.{$rawBody}";
$expectedSignature = hash_hmac('sha256', $payloadToSign, $secret);
return hash_equals($expectedSignature, $receivedSignature);

Payload formats change under you. The gateway evolved from a legacy {status, pk, reference} payload to a new nested {event_type, data.object}shape. The handler normalizes both, the webhook processor has to survive your provider's API drift.

A logging bug that could take payments down. The production log channel (Nightwatch) occasionally had a file-permission issue. Any Log:: call that threw inside the webhook handler would make the whole webhook fail with a 500, which the PSP would retry, repeatedly, potentially blocking legitimate payments. The fix: wrap the log in a best-effort try/catch so a log failure can never fail a payment.

try {
    Log::info("Webhook received ({$gatewayName})", ['payload' => $request->all()]);
} catch (\Throwable $logE) {
    // silent, a log write failure must never block a payment
}

What this taught me

Payments demand a specific engineering temperament. The interesting bugs aren't on happy paths, they're in retries, races, and provider drift. Three rules I now apply everywhere:

For a legaltech, where trust isthe product, the escrow isn't a feature. It's the whole point. And it only works if the code underneath refuses to let money flow twice, or to let a log line take a payment down.