Your loan officer will help you determine what purchase price you can get approved for. And also how much down payment you'll need, plus closing costs. Once you have an estimate of how much money you'll have to bring to the closing of your new home, you'll know what the bottom line is for the sale of your existing home. In other words, if it's determined that you need $10,000 to close on your new home, then you know you must walk away with at least $10,000 in proceeds (after payoff of current mortgage, realtor commissions & closing costs) from the sale of your existing home. Hope that makes sense. Good luck!