A colleague of mine received two postdoc job offers from two universities in the same city. He finds the two projects very interesting. He made his decision and now is drafting an email to reject one of the offers.
The issue is that we live in a small city and everyone knows each other, so he needs to be very delicate with his email. In fact, some research staff members are working with both groups at the same time. My colleague could suggest a collaboration to work on both projects, but he says he prefers to stay focused on working on one project. So he wants to reject one of the offers, but he wants to keep the door open in the future as he may want to work with the group in the future.
The question: What can my friend say in the email to show he sincerely regrets rejecting the offer, and that he is open to having collaboration in the future?