Is it lawful for my father to sell the land to others after the original purchaser failed to pay the balance?

In the year 1985 my father purchased some land with two other people as partners, after some months he sold it to other person in year 1986, for Rs 25,000 and the person who purchased the land did not have any written agreement, he just told that he will pay the said amount and get registered on his name. And my father paid his partners what they had to get from his pocket because the purchaser was paying the money very lately. In 2002 he paid some amount and said that he still had to pay 5800 and then he will get registered. But till now he did not paid the amount nor he got registered. And my father told him many times to pay the amount soon but there was no response. Two months back my father went his house and told him if he wont pay the balance he will sell the land to others, he did not cared. And my father sold the land to others for 4 lacs (today’s value of the land) and the buyers of the land get registered on their name. What my father did is lawful or not?