Legal Question in Real Estate Law in Maryland

You may be asking 2 different questions. If you refinance a property you may have to pay RECORDATION tax on the difference between the old and new mortgage. That is assessed when the loan is made, not when the property is sold. Separately, if you sell a property for more than you bought it for, you may need to pay CAPITAL GAINS tax on the difference between your basis in the property and the price you sell it for. The amount of the mortgage really doesn't factor into the equation.

While it is good to seek personalized accounting / legal advice for any question like this, generally speaking "basis" means the price one paid to buy the property plus what one paid for capital improvements. Things like deprecation will also affect the basis. In some cases a refinance pays for an addition or other improvements which might get added to the original purchase price to figure out basis.