You have the following option:-
1. To get issued notice asking the sellers for execution of sale deed and if they donot comply then file suit for specific performance of agreement to sell ,
2. To issue notice and demand back the paid part consideration along with interest and if sellers do not comply with the notice then file suit for recovery along with cost and interest.
In the light of given facts, option first is the best remedy. At the time of subsequent charge created by seller is malafide and the liability vest with seller alone.
In case of need consult with details.