I didn't know there was anything in butane to worry about. The butane itself would evaporate 100%, and the stinky hydrogen sulfide they add to ID leaks would also evaporate very quickly.
I guess it isn't reagent-pure, and some contaminants in the butane might be toxic and non-volatile. However, if they are water-soluble and don't bind strongly to soil, they would wash out over time.
http://www.pesticideinfo.org/D...
One approach might be to improve the soil gradually with compost and cover crops, and/or grow ornamentals for a while, then declare the area safe for food crops. Or get a soil sample analyzed, after asking the lab if they test for any contaminants that might have been in butane.
My yard is so small that every sunny square foot I can use is not to be wasted. But if you have as many beds as you need, using even a shady patch might not be necessary.
