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Incorrect fiat-to-msat conversion causes wrong Lightning invoice amounts in manual and auto-pay billing #6

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opened 2026-04-11 13:32:21 +00:00 by userAdityaa · 0 comments
userAdityaa commented 2026-04-11 13:32:21 +00:00
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Summary

Billing converts Stripe invoice amount_due directly to Lightning msats using amount_due * 1000, which treats fiat minor units as if they were sats-derived units. This makes paid amounts market-dependent and generally wrong, because no FX quote is applied.

Business impact

  • Underpayment or overpayment risk on every Lightning payment depending on BTC/USD at payment time.
  • Auto-pay path can mark Stripe invoice paid out-of-band after paying the wrong Lightning amount
  • Financial reconciliation drift between Stripe fiat invoice ledger and Lightning receipts.

Why this is definitely wrong

  • Stripe amount_due is fiat minor units, not sats or msats.
  • NWC make_invoice amount expects msats.
  • Multiplying cents by 1000 assumes an implicit fixed FX rate between fiat and BTC, which does not exist in code and changes continuously in reality
### Summary Billing converts Stripe invoice amount_due directly to Lightning msats using amount_due * 1000, which treats fiat minor units as if they were sats-derived units. This makes paid amounts market-dependent and generally wrong, because no FX quote is applied. ### Business impact * Underpayment or overpayment risk on every Lightning payment depending on BTC/USD at payment time. * Auto-pay path can mark Stripe invoice paid out-of-band after paying the wrong Lightning amount * Financial reconciliation drift between Stripe fiat invoice ledger and Lightning receipts. ### Why this is definitely wrong * Stripe amount_due is fiat minor units, not sats or msats. * NWC make_invoice amount expects msats. * Multiplying cents by 1000 assumes an implicit fixed FX rate between fiat and BTC, which does not exist in code and changes continuously in reality
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Reference: coracle/caravel#6
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