I applied to several corporate research labs for internships. Seeing no progress on my application, I emailed a professor in a top university asking for an internship. He was impressed by my work and agreed to accept me. Now I got an offer from a top corporate lab in my area of research. How do I politely decline the professor's offer, given that I might want to do a PhD with him in future?
Just send her/him a polite e-mail saying that although you were/are very much looking forward to working with her/him, you are taking another offer right now, but wish to stay in touch:
Dear Prof. X, I am very much looking forward to working with you. However, at this time I have also received a placement offer at company Y, and I will go there. I hope we can work together in the future.
Or something similar. If you are polite and honest, you should be fine.