# Requirement change

A few of our groups are trip groups, and trip groups have a problem: somebody pays for the taxi
in the local currency, and everyone else in the group is thinking in their home currency. Right
now there's no way to log that expense at all without doing the maths yourself first and hoping
you didn't round it wrong.

**We want to log an expense in a foreign currency and have it convert automatically.** You'll be
given a conversion rate at the time — think of it as "for this trip, treat 1 unit of the foreign
currency as worth this many minor units of home currency." The app doesn't need to fetch a rate
from anywhere; it's handed one.

A few things worth being precise about:

- **The split still has to make sense in the original currency first.** If somebody logs an
  exact split that doesn't add up, that should be refused for being a bad split — not quietly
  "fixed" by the currency conversion into something that happens to add up in the wrong currency.
- **After conversion, everything is just home-currency money.** There's no such thing as a
  foreign-currency balance sitting around afterwards. Converting is a one-time step when the
  expense is logged, not something that happens every time someone asks about a balance.
- **The usual rules about rounding still apply.** If converting leaves numbers that don't quite
  add up to the converted total, close the gap the same way an uneven split always has been.
- **An expense logged the normal way is unaffected.** It always was, and still is, 1-to-1 in the
  group's own currency.
