How would you design a payment system?

// A marketplace payment flow on a double-entry ledger, in integer minor units (cents).
// Debits are positive, credits negative; every transaction must sum to zero.
const fmt = (c) => (c < 0 ? "-" : "") + Math.floor(Math.abs(c) / 100) + "." + String(Math.abs(c) % 100).padStart(2, "0");
const yn = (b) => (b ? "yes" : "no");
const pspFee = (amount) => Math.floor((amount * 29 + 500) / 1000) + 30; // 2.9% + 30c, rounded half up

console.log("== 1. Money is integers ==");
let f = 0;
for (let i = 0; i < 10; i++) f += 0.1;
let cents = 0;
for (let i = 0; i < 10; i++) cents += 10;
console.log("ten x 0.10 as float = " + f + ", equals 1? " + yn(f === 1));
console.log("ten x 10 cents      = " + fmt(cents) + ", equals 1.00? " + yn(cents === 100));
function allocate(total, parts) { // largest remainder: shares always add up to total
  const base = Math.floor(total / parts);
  const out = [];
  for (let i = 0; i < parts; i++) out.push(base + (i < total - base * parts ? 1 : 0));
  return out;
}
const split = allocate(10000, 3);
console.log("split 100.00 three ways: " + split.map(fmt).join(" + ") + " = " + fmt(split.reduce((a, b) => a + b, 0)));

class Ledger {
  constructor() { this.entries = []; this.bal = new Map(); this.ver = new Map(); this.seen = new Set(); }
  balance(acct) { return this.bal.get(acct) || 0; }
  version(acct) { return this.ver.get(acct) || 0; }
  post(txId, legs, expect) { // expect: optional {account, version, minCredit}
    if (this.seen.has(txId)) return "duplicate " + txId + " ignored";
    if (legs.reduce((s, l) => s + l[1], 0) !== 0) return "rejected " + txId + ": legs do not sum to zero";
    if (expect) {
      if (this.version(expect.account) !== expect.version) return "rejected " + txId + ": version conflict on " + expect.account;
      const after = this.balance(expect.account) + legs.filter((l) => l[0] === expect.account).reduce((s, l) => s + l[1], 0);
      if (after > 0) return "rejected " + txId + ": " + expect.account + " would go negative";
    }
    this.seen.add(txId);
    for (const [acct, amt] of legs) {
      this.entries.push([txId, acct, amt]);
      this.bal.set(acct, this.balance(acct) + amt);
      this.ver.set(acct, this.version(acct) + 1);
    }
    return "posted " + txId + " (" + legs.length + " entries)";
  }
}

console.log("\n== 2. Order ORD-1: buyer pays 100.00, platform keeps 10% ==");
const L = new Ledger();
const fee1 = pspFee(10000);
console.log("processor fee on 100.00 = " + fmt(fee1));
console.log(L.post("ORD-1:charge", [["psp_receivable", 10000], ["seller_payable:S1", -9000], ["revenue:commission", -1000]]));
console.log(L.post("ORD-1:charge", [["psp_receivable", 10000], ["seller_payable:S1", -9000], ["revenue:commission", -1000]]));
console.log(L.post("ORD-1:fee", [["expense:psp_fees", fee1], ["psp_receivable", -fee1]]));
console.log(L.post("ORD-1:settle", [["cash:bank", 10000 - fee1], ["psp_receivable", -(10000 - fee1)]]));
console.log(L.post("ORD-1:bad", [["cash:bank", 500], ["revenue:commission", -400]]));

console.log("\n== 3. Partial refund of 30.00 ==");
console.log(L.post("ORD-1:refund", [["seller_payable:S1", 2700], ["revenue:commission", 300], ["cash:bank", -3000]]));
console.log("seller S1 is owed " + fmt(-L.balance("seller_payable:S1")));

console.log("\n== 4. Two payout requests race for the same balance ==");
const v = L.version("seller_payable:S1");
const owed = -L.balance("seller_payable:S1");
const payout = (id, ver) => L.post(id, [["seller_payable:S1", owed], ["cash:bank", -owed]], { account: "seller_payable:S1", version: ver });
console.log("both read balance " + fmt(owed) + " at version " + v);
console.log("worker 1: " + payout("PAYOUT-1", v));
console.log("worker 2: " + payout("PAYOUT-2", v));
console.log("worker 2 re-reads version " + L.version("seller_payable:S1") + ": " + payout("PAYOUT-2", L.version("seller_payable:S1")));

console.log("\n== 5. Trial balance ==");
let total = 0;
for (const acct of [...L.bal.keys()].sort()) {
  total += L.balance(acct);
  console.log("  " + acct.padEnd(20) + fmt(L.balance(acct)).padStart(9));
}
console.log("  " + "sum".padEnd(20) + fmt(total).padStart(9) + "   entries: " + L.entries.length);

console.log("\n== 6. Daily reconciliation against the processor report ==");
L.post("ORD-3:charge", [["psp_receivable", 4000], ["seller_payable:S2", -3600], ["revenue:commission", -400]]);
const report = [["ORD-1", 10000, pspFee(10000)], ["ORD-2", 2500, pspFee(2500)]];
const ours = new Map();
for (const [tx, acct, amt] of L.entries) if (tx.endsWith(":charge") && acct === "psp_receivable") ours.set(tx.split(":")[0], amt);
const theirs = new Map(report.map((r) => [r[0], r[1]]));
for (const id of [...new Set([...ours.keys(), ...theirs.keys()])].sort()) {
  const a = ours.get(id), b = theirs.get(id);
  const verdict = a === undefined ? "MISSING IN LEDGER (lost webhook?)" : b === undefined ? "MISSING AT PROCESSOR" : a === b ? "matched" : "AMOUNT MISMATCH";
  console.log("  " + id + "  ledger " + (a === undefined ? "-" : fmt(a)).padStart(7) + "  report " + (b === undefined ? "-" : fmt(b)).padStart(7) + "  " + verdict);
}
console.log("processor fee on 25.00 = " + fmt(pspFee(2500)));