Property bought without registration, owner claims for more money

Greetings for the day! My father has purchased a land belonging to his elder brother, who have two sons. The problem here is that- there wasn't any registration done for that keeping trust in the owner. Initial payment was done and remaining due was supposed to be paid later, which was agreed by both sides and documented. Now his son claims for more money for the same land although in the past he agreed for the amount fixed. There are about 7 months in hands for registration post to which it will be void. But the B party is not ready for registration until unless we agree for the new amount;he is claiming. Local village heads are in his support and asking us to increase the amount without any logic. Land papers are with us currently. A warining was given to them stating that either we will give the land back to them and pay us as per legal terms or we will file a petition.They are dragging this even thereby trying to make the registration impossible as there are only 7 months left. Please suggest further how to proceed. Thanks in advance.