Mushroom compost salts are highly soluble, and leach away pretty readily. Just stick a pile somewhere it gets rained on, and it'll be fine in a year. I believe that the nitrogen content may be quite high however (incomplete composting), so it can burn seedlings. Again, setting the stuff aside for a year should fix that.

Try it and see. I agree with above that after a few months outdoors and certainly a year should be fine. If it were me, I'd get as much as I could load up.

Perhaps this article about spent mushroom substrate may dispel some of the myths about the "salts" in the stuff.

https://plantscience.psu.edu/research/centers/turf/extension/factsheets/mushroom-soil

Excellent link kimmq thank you. I too am not sure where all these 'salts' would come from. From the link:

Mushrooms are grown in a mixture of natural products, including horse-bedded straw (straw from horse stables), hay, poultry manure, ground corn cobs, cottonseed hulls, gypsum, and other substances. This mixture is composted in piles or ricks, creating a dark brown, fibrous, and pliable organic growing media.

Sounds like compost to me. Yes, some of the ingredients might contain more soluble salts than others (horse bedding and poultry manure) but there are other ingredients, and again, the stuff is composted somewhat before being used. I would not worry about salts. Note, BTW, that NPK and other nutrients are in the form of salts. Unless you're in a super arid environment and fighting high salt levels already, it should not be an issue.

A friend of mine set up brand new garden beds with mushroom substrate and had great success, with no waiting period. But if you're worried about how fully composted it is, or anything else, till it into the soil in the fall and it'll be ready to go in spring. No need to park it for a year IMO.

I had a mushroom farm near me that unfortunately closed a few years ago. I used the compost when I took out sizable areas of lawn and put in perennial borders, i applied about 6 inches and tilled it into my fast-draining sandy loam. No problems that I was aware of.

I'd suggest that you find out what kinds of pesticides they use.