Money going out and money coming in, worked out in the open: an invoice with a register that knows what's overdue, and a loan schedule where interest is computed month by month so you can see it.
Amount, rate, term, first payment: the monthly payment and a full schedule where interest is computed month by month from the balance, rounded to cents like a lender, so every figure is visible. Pro adds extra payments with payoff date and interest saved, a rate-and-term comparison grid, and a formulas walkthrough.
An invoice that adds itself up and prints on one page: line items, discount, tax, total, due date from terms. Pro fills the client's details and item prices from lists, and keeps a register of every invoice — outstanding, days overdue, status, aging buckets, next number. No macros.
Bill is an invoice that computes subtotal, discount, tax and due date, prints on one page, and — in Pro — keeps a register of every invoice with outstanding amounts, days overdue, status and aging buckets. Principal is a loan schedule of up to 480 payments that stops itself at exactly zero, with extra payments, payoff date and interest saved in Pro.
Both round where money is rounded: discount and tax to cents on the invoice, interest to cents each month on the loan, with the last payment absorbing the odd cents. The Formulas sheets show where.
The main sheet, with a credit line kept. Pro adds the register, the lists that feed the dropdowns, extra payments, the comparison grid and the formulas walkthrough.
In Pro, yes: pick the client from a dropdown and the address, email and payment terms fill in from the Clients sheet. Items work the same way with a default price you can overtype.
IPMT gives the right number and no idea why. Computing interest from the previous balance each month keeps every step visible and makes extra payments a one-column change.
The invoice sheet, or the loan sheet with the full schedule, with a credit line kept. Pro adds the register, clients, items, extra payments, the comparison grid and the formulas walkthrough.