I agree with Jose, short sales have been done with no problems after a loan modification but it does depend on your bank's cooperation. I would recommend that you not only look over your loan modification papers, but you should also contact an expect real estate agent who works with short sale attorneys and deals with short sales. Some banks have information on websites explaining how their short sales work so you should check out your bank's website.
We've done loan modifications and then short sale the property without a problem, but it all depends on your financial situation and the bank's cooperation. As you know most of the modifications have re-defaulted.