Am I sinful, and must I seek absolution or pay compensation to the bicycle owner, for using the bicycle for an additional time after the agreed-upon rental period ended, with the lessor's knowledge of that?

The rental agreement is not valid unless the benefit is known, and it is known either by the duration or the work. If the rental period ends, the renter is not permitted to utilize the leased item except with the permission of the lessor. The questioner must seek forgiveness from the owner of the bicycles or pay the rent for the period exceeding the agreed-upon time. The renter is obligated to pay a fair market rent (ajr al-mithl) if he occupies the property after the rental period.