Mattress disposal

Take the mattress to the dump. Then take the money that it would have cost to industrially clean it, and give it (and some more to buy another mattress) to a homeless charity.

I agree with Anon.
You are extremely unlikely to find anyone who will take a used matress no matter what state it is in.

I am not sure about charities but you'll be surprised what people take if donated. Not everyone in town is on an expat salary .

Post it as a free item in Facebook/Marketplace or look for groups "A donner" or something like that.

It will be gone in a day.