Sudah berhasil dengan cara seperti di bawah ini, terima kasih guys.


<script type="text/javascript" src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
<form method="post" action="" onSubmit="return validateForm(this)">
<input type="radio" onclick="setJumlahBayar()" name="option" value="Cash" > Cash
<input type="radio" onclick="setJumlahBayar()" name="option" value="Transfer" > Transfer

<input type="text" name="bayar"  id="bayar">
 <input type="submit" name="save"  id="save" value="save">
</form>
<script language="javascript">
function validateForm() {
  var bayar = document.querySelector('input[name="bayar"]');
  var jenisBayar = document.querySelector('input[name="option"]:checked');

  if (bayar.value === '' && jenisBayar.value === 'Cash') {
    alert("Anda belum mengisikan Jumlah bayar");
    bayar.focus();
    return false;
  }

  return true;
}

document.querySelector('form').addEventListener('submit', function() {
  return validateForm();
});


function setJumlahBayar() {
  var jenisBayar = document.querySelector('input[name="option"]:checked');
  var data = {
     "Cash":'' ,
     "Transfer": '0'

  };

  document.querySelector('input[name="bayar"]').value = data[jenisBayar.value];
}

document.querySelector('input[name="option"]').addEventListener('change', setJumlahBayar);
</script>