gist

async sendTransaction(contractAddress, privateKey, from, to, amount, callback) {

    let sendAmount = web3.utils.toWei(amount.toString(), 'ether')

    const consumerContract = new web3.eth.Contract(config.erc20ABI, contractAddress);
    const myData = consumerContract.methods.transfer(to, sendAmount).encodeABI();

    const gasPriceGwei = 3;
    const gasLimit = 3000000;

    const [chainId, nonce] = await Promise.all([web3.eth.net.getId(), web3.eth.getTransactionCount(from, 'pending')]);
    console.log('chainId: ' + chainId + ', nonce: ' + nonce);

    const tx = {
      from,
      to: contractAddress,

      gasPrice: web3.utils.toHex(gasPriceGwei * 1e9),
      gasLimit: web3.utils.toHex(gasLimit),
      value: '0x0',

      gas: 0,
      chainId: chainId,
      nonce: nonce,
      data: myData
    }

    const rawTx = new Tx.Transaction(tx, { chain: 'mainnet', hardfork: 'petersburg' });
    const privKey = Buffer.from(privateKey, 'hex');
    rawTx.sign(privKey);
    const serializedTx = rawTx.serialize();

    console.log(`Attempting to send signed tx:  ${serializedTx.toString('hex')}\n------------------------`);

    const receipt = await web3.eth.sendSignedTransaction('0x' + serializedTx.toString('hex'),
      function (err, hash) {
        if (err) {
          callback(err, null)
        }
        callback(null, hash.toString())
      })
    console.log(receipt)
  },
}