A clear indication that the amount may change would be definetely warranted.
Following our lengthy discussion about Foreign currency transactions last month I was all excited about using Mondo to pay a bill instead of Paypal - A$1000 => £515 with paypal or £482.21 with Mondo. That was great - 35£ cheaper than Paypal. Submitted expenses straight away with a screenshot of the transaction to justify the exchange rate.
Except the transaction has now cost me £485.57… sure it’s only £3.26 difference - still far better than paypal - but can’t really get the difference back now that the paperwork is submitted. A warning would have saved me from submitting so fast