Hello Raghavendrahari.N, I acknowledge your question. The buyer here is filing a suit of Specific Performance and it is duly valid for him to do so. In this the buyer has proved his readiness and willingness through paying the token money as consideration, he shall duly pay the rest consideration therein after the registration of the agricultural land.
And when coming to the point that he not paying the additional consideration you can even prove that the above act would lead to his fault in showing the readiness and willingness in performance of the act.
The question as to whether he can seek for registration through a suit filed is valid according to the judgement held in SMS Teas Estates Pvt. Ltd. v, Chandmari Tea Co Pvt. Ltd.
The judgement held that the non-registration of the lease deed did not affect the invocation of an arbitration agreement and the validity of an unregistered document shall last till 3 years.
I hope that I have answered your query, if you have any further doubt do drop here.
Have a good day.