Is Notarised document with a back date of 6 months valid legally?

Sir, We have initiated an Agreement on a Rs 100 stamp paper, with a person who claimed, he is the sole authority to provide a flat against stipulated consideration to us in mumbai. This has been notarised having a back date of (6 months) Feb 2015. This person is supposed to take charge of the proceedings and the documents and claims to have the sole authority to finalise the documentation of the said flat for sale. We are or rather supposed to be the 1st prospect buyer for the same, according to him. Based on this, we released an initial part payment of Rs 975000/- by cheque which is notarised to the concerned person as follows: 1. 25000/- 2. 350000/- 3. 600000/- Now the concerned person is asking us to pay 24 Lakh after which he will issue us the 'Offer / Allotment Letter'. The rest will will be paid after receiving the Sale Deed Agreement in hand. This will be done via loan from bank. The initial amount paid, Rs 975000/- has been typed on a stamp paper of Rs 100/- and Notarised. But the Stamp paper has a back date of 16th Feb 2015 stamped on it. While the agreement has been typed and notarised on 28th August 2015. This means, a difference of 6 months. Will this pose any problems in legal terms. Is the Notary valid where the stamp paper has been back dated for 6 months. Will it carry weightage for legal proceedings in future. Please advice. Note: Is the document which is Notarised with a back date of 6 months court stamp paper of Rs 100 valid in the court of law in case of legal disputes. ? Thanking you.Gomes **