Senior Software Engineer - Money Movement

Simplex • New York City, NY

4d • Onsite

About The Position

This role is on-site M-F in the Flatiron District, New York City, NY. Our client is a Series A point-of-sale software company in NYC. We move fast and ship daily. AI-assisted engineering isn't a talking point here; agents like Devin, Claude, and Codex are part of how the team works every day, and engineers who master that leverage ship multiples of what they could alone.

About the Team
Money Movement owns everything that touches money after the sale: Payroll , Banking , and Lending . Today that means the accounting backbone our merchants reconcile their businesses on — QuickBooks sync for invoices, bills, COGS journal entries, and sales mappings — plus the ledgers behind loyalty, gift cards, and deposits, where being off by a cent is a bug. Next, it means payroll: our merchants run stores with hourly staff across multiple locations, and they want to pay their people from the same system that already knows every shift and every drawer count. This team is where we go from recording our merchants' money to moving it.

About the Role
You'll be one of the first engineers on our payroll build, and a technical owner of the financial systems where correctness is non-negotiable: ledgers, accounting sync, and the money movement rails underneath them. This means designing systems an auditor could read — double-entry ledgers, idempotent payment execution, reconciliation that proves everything ties out — and debugging from ambiguous production signals ("this merchant's books are off by $0.47 since the 9th") down to root cause. You'll partner closely with Platform Engineering, whose sale write path your ledgers consume, and directly with store owners whose payrolls you'll run. Beyond building, you'll establish the financial-correctness patterns the rest of engineering adopts.

Requirements

• 5+ years building production backend systems, ideally in TypeScript/Node — though we care more that you write correct, reviewable code in any language
• Practical fluency with transactional guarantees: idempotency, atomicity, retry semantics, and what happens when a webhook arrives twice
• Real depth in PostgreSQL: schema design an auditor could follow, transactions under concurrency, and queries that stay fast as ledgers grow
• An instinct for the unglamorous parts of money — reconciliation, rounding, tax rates with three decimals — done exactly right
• A bias for root cause over patch, and a track record of turning one-off incidents into eliminated bug classes
• Comfort operating with high ownership in a small team where your decisions become the standard

Nice To Haves

• Experience with payroll, banking, ledger, or other financial systems where an auditor has actually read your data model
• Experience with accounting integrations (QuickBooks, NetSuite, Xero), payment processors, or ACH rails
• Familiarity with our stack: Next.js, tRPC, Prisma, Vercel, ElastiCache, RDS/RDS Proxy
• Experience directing AI coding agents (Devin, Claude Code, Cursor) and building the guardrails that make them safe at scale
• Retail or POS domain experience, especially multi-location operations or cash reconciliation
• History of establishing technical standards or architectural principles adopted across teams

Responsibilities

• Build our payroll product from the ground up — earnings computed from shift data the platform already has, tax withholding, payment execution, and filings integrations
• Develop our bill pay product that moves our customers from paying their suppliers by check to one-click handling accounts payable directly in our platform
• Embed bank accounts directly into our OS so that our customers have access to their card processing revenues in minutes
• Own and harden our accounting integrations (QuickBooks today), where a sync bug shows up as a merchant's books not balancing at month-end
• Design and maintain the ledgers behind loyalty, gift cards, and deposits — correct, idempotent, and auditable under concurrent writes
• Establish the financial-correctness patterns (double-entry ledgers, idempotency keys, reconciliation jobs, timezone cutoffs) that other teams adopt
• Multiply your output with AI agents: direct, review, and productionize agent-written code as a core part of your workflow — with the guardrails money-touching code demands

Benefits

• Fully company-covered medical and dental benefits.
• 401(k)
• Catered Lunches and Dinners: We provide dinner each evening for employees who would like to eat and connect with the team. Lunch is provided on a weekly basis.

What This Job Offers

• Job Type: Full-time
• Career Level: Senior
• Education Level: No Education Listed