1) you can cancel the agreement on grounds of failure to pay balance amount within period of 3 months as specified in agreement for sale
2) refund the advance amount received by cheque .
3) take receipt from the purchaser that sum of Rs 2.50 lakhs has been received
4) in respect of flat booked by you cancel your agreement and request seller to refund your advance money . i presume there is no clause of forfeiture of advance money