How can one atone for using money donated for treatment after its need has passed, and is it sufficient to donate its value instead of returning it to him, and is this act considered usury?

Your father must inform the owner of the money that your need for medical expenses has ceased. He is obligated to return any amount paid to you, completely, once it is no longer needed, if requested. It is not permissible to remain silent about this. If any of the money has been spent, its equivalent must be returned, and giving it as charity is not sufficient if it is possible to deliver it to its rightful owner. As for what was purchased with the money, it is permissible to benefit from it.