Some Financial institutions are introducing a mortgage product where you can carry your shortfall onto the next property. for example, if you have a property with a mortgage of 100k and you sell for 90k, the 10k shortfall you can carry onto your next purchase. so, if you pay say 130k for the next property, it will really be 140k in the banks eyes as you will be carrying the 10k shortfall from your current property.
If you simply intend to sell and not purchase immediately, you will be liable for the shortfall and you will have to come to some agreement with the bank to repay it. They should be willing to allow you to sell but again it will all be down to the outstanding balance and what means you show to prove you will be able to repay it i.e. not leave the country