bought a house and lot without deed of sale but had receipts for payment

a house and lot was sold to us but we weren't able to have a conditional deed of sale made..well it's not yet ours but it is still being paid installment. the owner gave us a price and we agreed verbally because my parents trusted her and we started paying and we paid more than half. two weeks ago he came to us and said that she will increase the price but we agreed previously to a certain price already and we had acknowledgement receipts for the payments but unfortunately there was no conditional deed of sale since my parents trusts the owner very much. my parents are ready to move in this month and now this happens. what can we do?