Just like any other agreement
It should be fairly simple. Just make sure you have a written and signed agreement in place with your dealer. Also, I would get it signed by the sales manager, not your sales person. Make sure the agreement states that you will get a particular car number from all the M3s they get and that your deposit is fully refundable. Some folks have even got their dealers to agree to refundable after they spec and order provided it is not some off-the-wall color/option package. Good luck.