The determination of the residual debt at a given date of a lease agreement, when it occurs the case of insolvency or continuous arrears (i.e. an early termination, before the maturity of the lease plan), is often regulated by the contract, which fixes penalties and some kind of impairment reimbursement. Both the lessor and the lessee are required to calculate separately for the amount of the outstanding debt and the sum of the impairment reimbursement and of the penalties. In this paper, the authors propose a model for a precise quantification of the residual debt, the damage impairment and the penalty shares based on three rates: the contractual rate, the implicit internal rate(s) of return and the market prime rate. This model is consistent with both finance and accounting perspectives. The developed methodology can also be proven capable to detect early any usury behavior, when it is given a threshold by the law or when it can be inferred from the market, therefore improving decision making and the forecasting of actual costs of the agreement.