code: async function sendApproveZero() {
  
    console.log('Starting erc20 outbound', opts);
  
    // Get the raw approve tx with 0 value
    const approveTx = cctx.buildApproveTx(Object.assign({}, opts, { value: '0' }));
  
    // Send the approve transaction on Wanchain
    const receipt = await utils.sendRawWanTx(web3wan, approveTx, opts.from, wanPrivateKey);
  
    console.log('Token approval removed');
    console.log(receipt);
  }
docs: Note: An approval will fail if a non-zero amount is already approved for that account, in which case it is necessary to first zero out the approval.