If you sell your house for less than what you owe and have the money to make-up the shortfall, it is not a short sale (which occurs when you do not have any assets) and does not require the mortgage lender's approval, and therefore will not have a negative impact on your credit so you can purchase another house if you meet financing criteria.
If you do not have the difference in selling price vs. what you owe, you will need to talk with your lender.
If I can be of further assistance, please let me know. I've got experience with short sales and would be glad to help.
Call me and I can discuss options if you would like.
Thank you Lise Tremblay
Hope this helps!