Purchased land - Regarding.

Hi, I purchased a land in Greater Noida, from an army person in 2008 after that in 2014 when we visited the site randomly we saw someone started construction on that land , when asked he placed me before the 1st owner of the land ( so called the owner of almost all plots there & a person who sold that plot to the army personnel) he consoled us by saying that- since nobody acquired this piece of land from long time so he sold to someone else who approached him. When we asked about my concern he said not to worry we have another plot vacant at this point of time, which you can take , He swapped the land on a 100 rupees piece of paper with his signature. Now sir my concern is -- 1. Is this legal to swap lands? 2. Since the registry was between me and that army person mentioning details of that piece of land, I have a doubt on my right over this new land given to me. Am I ? 3. A Can i sell this property? 4. What papers i should i have to prove my rights over this land? Since i have 1st registry documents. Kindly suggest me with the legal aspects / ways to have rights and proper documents for this swapped land. Thanking you in anticipation. Regards.