Sale of Land subject to Repurchase

Good day! I just want to ask  some legal advice. My aunt and her husband,which I'll be referring to as the couple A, own a residential lot.Sometime in 1970s, another couple,which I'll be referring to as couple B, occupied a portion of the said residential lot with the permission of couple A. They said they're going to stay temporarily until they could find a place to move to. Years passed by but they still haven't left yet and they also refused to leave or pay for rentals. So my aunt and her husband (couple A) were forced to sell the lot portion to couple A for P35,000 with the condition of not selling it except to couple A.Couple B paid P30,000 as partial payment. Both couples never initiated to break the lot title. So as far as the lot title is concerned, the whole lot still belongs to couple A. Couple B did not contribute any amount for the real property tax. The remaining P5,000 was not paid until 2015 when my aunt was already mildly paralyzed and her husband was already dead. That amount is no longer equivalent to 1970's P5,000, considering the inflation. And since both of the couple B were already incapacitated due to old age,the said amount was paid by couple B's grown-up children and they haven't  given my aunt a copy of the receipt because they took it and said they're gonna ask the Barangay Captain to have it signed by him so my aunt wouldn't have to do it herself. Now we learned that couple B's children tried to sell the said lot portion to a particular person. The good thing is that the said person asked my aunt first about the details of the lot before buying it. So they were not able to sell it. As of this date, couple B are still alive but not in sound mind due to old age. My aunt is still in sound mind but mildly paralyzed. My aunt and her husband never had children. So she asked me and her siblings to settle the issue. Can couple B's children sell the lot portion to other people now that both couples lost their copies of their agreement? Can we forced them to sell it back to my aunt for just price,considering the fact that they did not contribute for real property tax and the effect of inflation on the full payment of P5,000?What remedies can we resort to?