Is it permissible to paint a ladder borrowed from a neighbor as a condition for its loan, and if not permissible, how should it be handled when returned unpainted?

Lending with a condition of recompense ceases to be a loan and becomes a lease (ijarah). If the lender stipulates a known recompense for a loan limited by a known period, it is a valid lease. However, if he stipulates a known recompense for a loan not limited by a known period, it is an invalid lease (ijarah fasidah), and in this case, the lender is entitled to the customary rent (ujrat al-mithl). For the lease to be valid in the presented case, the duration of use for the ladder and the type of paint must be specified.