Lot area sold has discrepancy, area decreased after re-survey

Thank you in advance for the legal advice. My situation is as follows.

1. I have the signed, notarized deed of sale. Lot was paid in full in 2013.
2. Lot sold to me was 300sqm, as per deed of sale.
3. Last month, the property seller (one of the big names out there) told me that after a "recent re-survey", the lot has decreased to 285sqm and that they want to refund me on this discrepancy.

I feel suspicious as this seems totally unnacceptable. I have already spent resouces in planning the lot area I thought I had. And it took them more than a year after the deed of sale to inform me of such discrepancy. The reason of the discrepancy, they told me, was that it was a "mistake" from the Land of Bureau. Also more suspicious to me is that the missing lot is just on the adjacent side, i.e. it now might be part of the lot a neighbor has.

Any advice on the matter is truly appreciated.

Salamat po!

-James

Ask ko lng, wla bang tax declaration or title indicating the boundaries, perimeters, etc. or describing the lot location? Kc in deed of sale the property being sold is described quoted from the title/tax declarations. I sold a lot recently, however, the buyer had it surveyed as she was suspecting the area indicated on the title seems wrong if just visualizing the lot. It was found out our neighborhood encroached 20 sqms, and instead of having a case in court, she and the neighbor agreed for a certain amount for the 20 sqms difference. The buyer had the title in her name changed to the correct area excluding the 20 sqms and I don't know if the neighbor had her title changed. Ask ko lng, could it be possible what happened in your case, the seller had it known that the neighbor encroached already the 15 sqms and had not changed/reflected it on his title that he sold to you? Could it be also possible that the seller is proposing to you just to refund/pay you back for the 185 sqms for settlement of the problem? Ask ko lng, 2013 pa ang deed of sale at nakatransfer na ba sa pangalan mo ang property? PLEASE NOTE I AM NOT A LAWYER, I AM ONLY SHARING TO YOU MY THOUGHT AND EXPERIENCE THAT MIGHT BE OF HELP TO YOU.