Which currency
owes the bank

Point 1The matching principle

The baseline wisdom: match the debt's currency to the asset's — dollar mortgage against dollar property means rent services debt in one currency, and the exchange rate touches only your equity slice. The property's dollar cash flows and dollar value move together with the dollar loan; the FX exposure is contained to what you actually invested.

Mismatched debt — borrowing yen against a Manhattan condo — doubles the FX position: the asset is long dollars while the liability is short yen, and a strengthening yen raises your debt's dollar weight exactly as your dollar asset buys fewer yen. The mismatch can pay brilliantly (cheap yen rates funded a generation of carry trades) and break brutally; it is a currency trade wearing a mortgage's clothes, and should be sized like one.

Point 2The real options on the menu

The rate-spread temptation deserves its honest frame: borrowing at home where rates run lower, buying US property in cash, looks like free spread — until the currencies move. The spread is compensation for FX risk, not a market inefficiency; taking it knowingly, sized to survive adverse moves, is strategy. Taking it because the rate looked cheaper is the classic uncompensated risk.

Point 3Servicing across currencies

Even matched structures leak at the edges: a dollar mortgage serviced from home-currency income converts every month — small, recurring FX transactions whose costs and rate drift add up across decades. The mitigations echo the currency guide: US rental income servicing US debt directly (the natural hedge rentals provide), dollar reserves buffering the conversion calendar, and conversion channels chosen once for cost.

The reverse flow matters at exit: mismatched structures unwind with the property's sale — the yen loan repaid from dollar proceeds at whatever rate reigns — making the exit's FX moment carry the accumulated position. Owners running mismatches should mark them annually: the loan's home-currency value against the property's, the position's current profit or loss made visible rather than discovered at sale.

Point 4Who should do what

The defaults by profile: the yield investor matches (dollar debt, dollar rents, contained exposure — the DSCR structures our mortgage guides map); the family buying use-assets matches or goes unlevered (the estate-tax debt benefits our cash-versus-mortgage article prices arrive with dollar debt anyway); and the sophisticated balance sheet with multi-currency assets and advisors may run deliberate mismatches — sized, marked, and reviewed like the positions they are.

The disqualifying answer is accidental exposure: the buyer who borrowed at home 'because rates were lower' without pricing the position, or who services dollars from yen without noticing the drift. Currency, like every subject in this library, rewards the decided over the defaulted — and the mortgage's currency line is a decision, whether or not it was made consciously.