The example below for your reference, when use enter the value in emp_name field and blur, the value will auto fill in the balance leave field.
Add the code below into script editor web part in new form page in list2.
<script src="https://code.jquery.com/jquery-1.12.4.min.js" type="text/javascript"></script>
<script type="text/javascript">
$(function () {
var listname = "list2";
$("input[title='emp_name']").blur(function(){
var url = _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getbytitle('" + listname + "')/items?$filter=emp_name eq '"+$(this).val()+"'";
$.ajax({
url: url,
method: "GET",
headers: { "Accept": "application/json; odata=verbose" },
success: function (data) {
if(data.d.results.length>0){
var item = data.d.results[0];
$("input[title='balance leave']").val(item.balance_x0020_leave);
}
},
error: function (data) {
}
});
});
});
</script>


If the emp_name field is a lookup field, we need use the code below.
<script src="https://code.jquery.com/jquery-1.12.4.min.js" type="text/javascript"></script>
<script type="text/javascript">
$(function () {
var listname = "list2";
$("select[title='emp_name']").change(function(){
var url = _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getbytitle('" + listname + "')/items?$filter=emp_name eq '"+$(this).val()+"'";
$.ajax({
url: url,
method: "GET",
headers: { "Accept": "application/json; odata=verbose" },
success: function (data) {
if(data.d.results.length>0){
var item = data.d.results[0];
$("input[title='balance leave']").val(item.balance_x0020_leave);
}
},
error: function (data) {
}
});
});
});
</script>