Private Lending Platform - Funding Installment Plans with Investor Capital

Four things had to be right. Matching: new invoices keep arriving from the seller's own system, and each needs an investor with enough free balance in the right currency, chosen by clear rules. Interest: every investor has to see how each payment was worked out, and rounding can't create or lose a cent. No double posting: the automation runs every few minutes, and a job that runs twice must not post the same payment twice. Visibility: investors need to see where their money is, what it has earned and what is due back, without asking.

Each investor has one wallet per currency, showing free balance, amount invested and interest earned, with deposits recorded by administrators and the option to put the account on hold or reinvest earned interest. A scheduled job pulls new installment orders from the seller's API, resuming from where the last successful run stopped, and turns each into an invoice with its down payment and schedule. For each invoice the platform finds active investors whose wallet covers the amount and picks by administrator-set priority; administrators can assign by hand and investors can take open invoices themselves. The interest engine splits the financed amount into equal parts of principal, computes the investor's interest on the declining balance for the days between installments, puts any rounding remainder on the last installment so the parts always add up, and records the spread as platform income. Every movement of money is a transaction in a ledger: funding and each repayment are posted as a mirrored pair, each entry carries a key built from investor, seller, invoice and payment, and the database enforces that the key is unique. Scheduled jobs confirm repayments and credit wallets; cash-out requests are checked against free balance including open requests. Investors get monthly statements, upcoming repayments and interest earned; administrators get dashboards over any period and reports on any investor or seller. The web app is built in Svelte, with an API client generated from the OpenAPI description and routes generated from the pages themselves, each declaring the roles that may open it.

In production, with investors funding installment plans through the platform. One ledger for deposits, investments, repayments, payouts and fees in two currencies, and intake, matching and repayment automated end to end. The ledger design, mirrored entries with a database-enforced idempotency key, is the pattern we reuse whenever software holds money.