How would you go about doing this? As an example, if you loaned someone 167 monero to buy a car and expect them to pay you back in 7 years like a bank does you would be requesting 167xmr*6.02% (to counter xmr inflation) for a total of 177.053xmr. 177.053xmr/84 (months in 7 years) would be 2.107xmr a month. At the moment that is fine, but if the usd price of monero rises and the borrower is being paid in usd then they are going to default and you will loose the xmr. The only way I could see to counteract this would be to lower the Monero payments per month, but then that would take even longer to be repaid.
Typically in multi-currency contracts, the payment schedule is denominated in the same currency as the asset. So if the car is purchased in Vietnamese dong, the repayment schedules denoted in Vietnamese dong. You could accept xmr for each individual payment, with some specification to the conversion rate.
If you don’t denominate the payments and the asset in the same currency, you run into situations where one moves and the other doesn’t, and one party is left holding the bag. So if you believe XMR is going to go up you are incentivized to denominate the repayments in XMR, but if you bet poorly, you better have a hedge available.
Most jurisdictions require payments to be accepted in the local currency. So even if you specified an XMR repayment rate, the person could still pay you in the local currency.
Not to mention when you add foreign exchange to transaction now you’ve got weird incentives going on. If XMR crashes, one party would not want to get paid in XMR. And if XMR rises too much, they would prefer to default and give you the vehicle instead of paying you the XMR.