Finance Lease Interest Expense

After reading the CFAI book and taking a Schweser mock exam, I have run into an apparent contradiction. When calculating the interest expense for a finance lease (lessee), in the CFAI material the interest expense is calculated as discount rate x (Beg. lease liability - lease payment). The Schweser mock simply used discount rate x lease liability. However the CFAI did specify that lease payments were made at the beginning of the period, while the Schweser mock did not specify. Does anyone know the correct way of calculating this, or does it depend on if the lease pmt is made at the beginning or end of the period?

Thank you

Both are correct. Let me try to put it:

Interest = discount rate + lease liability from the beginning of the period. (the liab. that accrued interest over the period)

in CFAI book: because they make the payment at beg of period, that effectively reduces the balance to the amount = payment. Therefore they do it that way

Scheweser states it in general case. Nothing is wrong with it