Write Re-hire Letter

I'm a supervisor and I had an excellent engineer was working with me for almost 20 years. 3 months ago he submitted his resignation due to better offer that was attracted him. However, he called me this week to check the possibility to re-hire again within our company which is required high level of management approval. Therefore, i would like to make an official letter using (Strong business/academic language) requesting our management approval to consider re-hiring him again. the letter should be using business format not like an email to be sent.