Partial Payment for Lot

Nagbigay kami ng partial payment for a lot amounting P500,000.00 sa land owner (seller).  Binigyan kami ng Acknowledgement Receipt stating the amount (P500,000.00) as partial payment for the lot signed by the seller herself. Nag-alala lang po kami kasi yong seller is already 72 years old, diabetic  at walang asawa at mga anak. Meron siyang mga pamangkin at  mga apo.  Baka hindi pa namin maibigay sa kaniya ang full payment, bigla na lang siyang kunin ni Lord at wala pang Deed of Sale.  Ano pong mabuting gawin namin upang maiwasan ang problema kung sakaling mamatay ang seller at wala pang Deed of Sale na completely signed by the seller herself and properly notarized?  At present, wala pa kaming pera for full payment.