Is the amount mentioned by the father to the questioner's mother — that the questioner should take it from her when he sells his land — considered an inheritance to be distributed among the heirs, knowing that the father did not state that it was a debt owed by the questioner?

Perhaps your father considered what he gave you a loan awaiting repayment, and what your mother mentioned is evidence that it was not a gift. Therefore, for the sake of your religion, it is safer to dispose of the amount by dividing it among the heirs, as it is a matter of doubt. And whoever guards against doubtful matters has safeguarded his religion and his honor.