I wouldnt ask for the ring. I would tell him firmly that as he hasnt proposed so far you want the ring back tomorrow, without fail. If he waffles and makes excuses, then I would say that if he doesnt produce the ring as requested, then that means he has stolen the ring from you and you will inform the police , and tell your daughter how long he has had the ring too. It is not pleasant having to tell someone that the person they thought was so marvellous is not to be trusted, and of course she may well decide that this alters things, and she doesnt want to marry him.
Although she will be upset by what you have to say, if you didnt tell her, then in the future she found out, that could affect your own relationship too. Be honest with her as you have always been and if that shows her that this man is not to be trusted she will thank you in the future. Whatever he does now you are very unlikely to trust him ever again.