To answer your question, I believe that buying down your mortgage will not affect your capital gains as the mortgage amount is irrelevant for tax purposes. Your CAPITAL GAINS are factored as follows...Current Sales Price - Cost Basis, which is Purchase Price (when you bought) OR market value at time when inherited (if inherited). I will not tackle dedcutions here as those are a separate entity.
As mentioned in previous answer, a 1031 "like kind" exchange would allow you to defer paying taxes on any capital gains that you reinvest.
Let me understand your question. First it appears you own vacant land that is secured by a mortgage and that you want to sell it to someone else. I am assuming you will need to payoff the mortgage to convey clear title to a new buyer. When you purchased the property I am also assuming you put some kind of downpayment along with interest payments. Both of these can be used as deductions before you pay any capital gains taxes. You can not depreciate Vacant Land.
In this market if you can find a buyer for vacant land you are in great shape!!