Effective-interest (annuity) amortization for a finance Leased instrument in Subscription Management

Summary:

Subscriptions are instantiated from CPQ quotes. A subscription line carries a financed “instrument” charge plus separate interest, service, and consumables charges, financed at a fixed rate over the term (e.g. instrument ~73,205, 7% APR, 60 monthly periods). The billing schedule produces a constant periodic invoice amount to the customer.

Current behavior
The financed charge is recognized straight-line – a flat principal and flat interest in every period – with the underlying asset amortizing linearly and NBV declining on a straight line.

Required behavior
We need the financed charge recognized on an effective-interest (annuity) basis: the periodic invoice total stays constant, but interest is computed on the declining outstanding balance and the principal portion is the residual – front-loaded interest, back-loaded principal – retiring the balance to zero at period 60. Asset amortization and NBV must follow the resulting principal-reduction schedule rather than straight-line.