need advice ON LAND CONFLICT

Sometime in 1963 my parents in law transferred to a parcel of land of more or less 900sq meter. In compliance with the landowners request to vacate their former abode to a better place with the understanding that the place shall be sold to them instalment basis. In 1964 with their new house constructed, we started living ther. Payments were made with receipts in the name of the landowner (now deceased)due to their inability to produce the needed title , payments to them were stopped. It was found out that the place, a former ricefield, with an irrigation canal running through it (which is a government property) is the main reason for the inability to produce the title.
In 1981, the place was left to us. The situation still the same, no title. In 2009 we, together with other residents in the place were sued for ejectment under rule on summary procedure. The case was dismissed on 2009.
This year we were surprisedto be informed that our lot has been mortage by one of the heirs, with whom I personally agreed top pay for the land were occupying if he could produce a title. However the oldest brother agreed to redeem the lot and sell it to us. Since we have initially pay some amount to their mother, how shall this be deducted from the price considering the big discrepancy in the value of the land then P50/sqm to P5000 they want now?