I am doing a loan modification. If the bank lowers my principal loan what will happen in the future if i want to sell my house?

It has no bearing on the future value of the house. The new principal is what you would owe to the bank, and the value of the home would be whatever the market is willing to pay for it at the time of sale.As for the profit, You would have to see if that issue gets addressed in your new mortgage contract. I can't see a scenario where the bank would just let you walk away with the money. I may be wrong, but I believe the bank would minimally try to recoup any money they forfeited to you in a modification in the case your home sells for more than the equity + principal up to the point of the original value of the home.