How about your spare, is it the same size? If not, then better buy a pair and then make the used 40% one as spare tire. Cuz the difference is just too much if the other one only left 40%. If still like 70-80% then still can.

Used tire with only 40% left will not get you anything much. Imagine that even tires that are still 80-90% condition are only priced around 25-35% of the new price, so yours being only 40% left probably can only fetch like 10% of the new price, that's IF they want to take it also! But you can try to ask them.

I suggest you buy a pair and put the used one as spare if you want.