They can and are. They're called "FX forwards", and what's more the technique you described is exactly the one the Medici family used to make their fortune (avo

They can and are. They're called "FX forwards", and what's more the technique you described is exactly the one the Medici family used to make their fortune (avoiding the usuary laws of the time). However it doesn't work if you can't guarantee a fixed rate of interest (as in this case).