It's really all relative. what you are losing on your condo you will gain when you purchase your new home at today's prices and the market starts to get stronger. It seems like you have enough resources to get into a single family home and that the need is there. Have a market analysis done on your unit and find out what your true numbers are. Then you can decide how much house you can afford. If I can be of any help don't hesitate to call.
I'm not aware of any mortage companies (especially these days) that would allow that. One option would be to purchase a home with only 5% down (using your example that would be $26,000) and using the remaining balance to pay off the balance of the mortgage when you sell your condo. So if you sell the condo for $245k and owe $265k you would pay $20,000 to your existing lender to pay it off at closing. That would still leave you with close to 10% as a down payment. Not the best option financially but if you feel you must sell the condo and get into a house this would work. Another option is to keep you condo and rent it out. Even if you couldn't get a full mortgage payment you could put 10% down on the new house and if you had to pay $100-200/month, you could reap the tax benefits and wait out for the market to rebound. Even if you had to wait 5 years for the value of your condo to increase you would have only paid $12,000 over the term (assuming a $200/month loss). But you would receive tax benefits for those losses and the tenant would be paying down your mortgage for you. Just an idea, best of luck.