How's this for a possibility-
The asking price of the field, may well have been be public knowledge (for sale sign), thus Peter and other nieghbours would have an anticipated amount fixed in their minds.
The couple sell the field for a major down payment, and extend credit to the buyer for the remainder. Peter, like the WTS, wants the money NOW.
The couple give him the full amount recieved, but can't commit to the portion on credit.
They may actually have been very poor and the greatest, real security of the poor is a piece of land -much the same even today.
They had no guarantee of the balance ever being paid - what if the buyer, died or fell sick, or cheated them? -all things beyond their control - and they had already given freely, ALL of what little security they had.
Pete knew the asking price of 10 and when he only got 7 or 8, he lost it, without all the facts - much like a modern day Judicial Committees.
GfG