1) FILE SUIT FOR SPECIFIC PERORMANCE OF CONTARCT
2) your friend had agreed to purchase your plot and paid Rs 5 lakhs as adavance
3)since he has fialed to pay balance amount of Rs 27.50 lakhs suit for specific performance is mintainable
4) also file police complaint of cheating and criminal breach of trust against frined under section 406, 420 of IPC
5) your friend would not get the ownership of land
6) adverse possession can be claimed as defence only . in your case friend agreed to purchase your land and suit is maintainable