Error 400, Stellar SDK JS

Question

I'm building an API to transfer assets from stellar based wallets, I am currently using the JS SDK. This is my code below:

var StellarSdk = require('stellar-sdk');
var express = require('express');
var router = express.Router();
StellarSdk.Network.usePublicNetwork();
var server = new StellarSdk.Server('https://horizon.stellar.org');

router.post('/sendToken', function (req, res, next) {
  var sourceKey = req.body.source_keys;
  var issuingKeys = StellarSdk.Keypair.fromSecret(req.body.issuing_keys);
  var receivingKeys = req.body.receiving_keys;

  var l2l = new StellarSdk.Asset('L2L', sourceKey);

  server.loadAccount(receivingKeys)
      .then(function (account) {
          var trusted = account.balances.some(function (balance) {
              console.log(balance);
              return balance.asset_code === 'XXX' && balance.asset_issuer === sourceKey;
          });
          if (trusted === true) {
              console.log('trusted')
              server.loadAccount(issuingKeys.publicKey())
                  .then(function (issuer) {
                    var transaction = new StellarSdk.TransactionBuilder(issuer, {
    networkPassphrase: StellarSdk.Networks.PUBLIC,
})
                          .addOperation(StellarSdk.Operation.payment({
                              destination: receivingKeys,
                              asset: l2l,
                              amount: req.body.amount
                          }))
                          .build();
                      transaction.sign(issuingKeys);
                      return server.submitTransaction(transaction);
                  })
                  .then(function (result) {
                      console.log('Success! Results:', result);
                      res.send(JSON.stringify(result));
                  })
                  .catch(function (error) {
                      console.error('Error!', error);
                  });
          } else {
              console.log('not trusted')
          }
      });
});

module.exports = router;

I have no idea where I'm going wrong, could anyone guide me?