The beginning receivable of the lease is equal to the present value of the rents, excluding executory costs, at the discount rate. For a lessor, this is the same as the implicit rate.
If the lease is finance, once you save the lease, the beginning receivable is calculated and displayed. You cannot set or change this number directly, and the amount does not change over the life of the lease, unless the lease is revised. It is a positive number, to indicate a debit.