Is a Muslim excused for forgetting the penalty clause in a sales contract, and is offering a quarter of the property in lieu of the penalty clause considered ownership, or is it contingent upon the other party's acceptance?

The penalty clause in financial contracts is permissible, except in explicit debts, according to the Islamic Fiqh Academy. The original principle for conditions is their validity and fulfillment, as Shuraih said: "Whoever imposes a condition upon himself willingly and without compulsion is bound by it." Your statement to your brother, "You have nothing with me," is considered a denial and repudiation of the contract, not a dissolution. Forgetfulness is excused, but it is not permissible to deny a right. As for your offer of "a quarter of the shop," nothing results from it if your brother does not accept it. And if you accept it, thinking it is an implementation of the condition, you are not obligated to give him a quarter of the shop because the condition is still in effect and you have not dissolved the contract. The questioner is advised not to pay attention to whispers (waswas), as they are a dangerous ailment that can corrupt one's religion and worldly life.