We did something like that. We both had talked about getting married, so we were both on the same page (this is very important). We both went and looked at rings, decided which we'd want and which sizes fit us, then I bought them and asked the jewler to engrave the date of the proposal. The wife wasn't allowed to see, obviously, so she only knew that it was going to happen *someday * but not when.

The date went great and she said yes. Everyone happy.

I recommend my colleague to do the same. He did not. Now he's trying to return the ring which is both too big and wrong style.