This code is working for me;
RestTemplate restTemplate = new RestTemplate();
Payment payment = new Payment("Aa4bhs");
MultiValueMap<String, Object> map = new LinkedMultiValueMap<String, Object>();
map.add("payment", payment);
HttpEntity<MultiValueMap<String, Object>> httpEntity = new HttpEntity<MultiValueMap<String, Object>>(map, headerObject);
Payment res = restTemplate.postForObject(url, httpEntity, Payment.class);