Please avoid such things. You will suffer. This is because
(1), it will be very difficult for you to handle the 12 lakh cash - especially when there is pressure on black money.
(2) giving two different agreements (i) one registered for Rs. 25 lakhs and (ii) another - unregistered for Rs. 37 lakhs will create problem.
(3) Once the property is registered for Rs. 25 lakhs, it is doubtful whether he will give you the Rs. 12 lakhs in cash. If he does not give, then you can never be able to recover the same, as according to the registered agreement, you would have received he full amount of Rs. 25 lakhs shown in that agreement.
(4) If one agreement for Rs. 25 lakhs is registered (as he can get loan only if the property value is in the range of Rs. 25 lakhs), what for he is requiring another agreement for Rs. 37 lakhs? Especially when that agreement cannot be registered.
WHATEVER MAY BE HIS PROBLEM, BETTER DO NOT INVOLVE IN SUCH TYPE OF TRANSACTIONS. AVOID. LOOK FOR SOME OTHER BUYER.