ureech,
переменные можно использовать, а вот кавычки будут всегда.
<script>
let name = "it_is_name", data = "it_is_data";
let ar = [{"sum":"40.00","title":"Еда"},{"sum":"40.00","title":"Еда"},{"sum":"100.00","title":"Акции"},{"sum":"1000.00","title":"Зарплата"},{"sum":"100.00","title":"Акции"}]
let keys = {}, out = [];
ar.forEach(({title, sum}) => title in keys ? keys[title].push(sum) : (sum = keys[title]=[sum], out.push({[name] : title, [data] : sum})));
document.write(`<pre>${JSON.stringify(out, "", 1)}</pre>`)
</script>