$payment = Payment::get($paymentId, $apiContext);
$execution = new PaymentExecution();
$execution->setPayerId($payerID);

$transaction = new Transaction();
$amount = new Amount();

$amount->setCurrency($currency);
$amount->setTotal($total);

$transaction->setAmount($amount);
$execution->addTransaction($transaction);
$payment->execute($execution, $apiContext);