A bit unsure of what you're asking....
If you got a loan modification approved already, and you still want to walk away....
Have you tried to do a short sale first? Although it takes several months to process, during that time, you can continue to live in the property while working out the sale. You have to prove hardship which can be a combination of many factors/reasons (loss of job, decrease in income, increased expenses, job relocation, family situations like divorce/separation/death, drop in market value, etc).
At least the effect on your credit won't be as devastating as a foreclosure or bankruptcy.
If you want to leave your place, you can do that while the house is being offered as a short sale. At least try that. There are agents who will be willing to work with you.
Another question for you: you identify yourself as both a buyer and a seller....are you buying a home now, and then walk away fron the one you have? If you're considering Buy and Bail, do note that is considered and treated as mortgage fraud.
Care to elaborate what you're thinking of doing?