My arrangement here is a deep litter straw bed in the barn and my hay supplier will dig it out, often for free and make a heap up the field, then bring a scoop or two back from the 2yo heap which I use for my garden and local gardeners to collect off the yard. Free that is, IF he can do it in his own time, which might be June through to December, depending on when it suits him. If I need it done to a particular timescale, then I expect to pay, which is usually an hour of his tractor time but he throws in travel time for that. And I try and add a hay/feed delivery on top which reduces his fuel costs for the job as he is 3 miles away by road. He doesn't always accept payment, but I always offer and the deciding factor is usually to do with how convenient the timing is for him.
I likewise don't charge folk to take away what I have in heaps, so long as they do it themselves. Even if I paid for the bedding and paid to have it dug out and stacked and returned back to the yard for collection, if they do the work they can have it, butI can't help load. Folk seem to come back and apparently the local village has a huge flatworm problem so they take my manure to restore earthworms to their gardens and always seem to find plenty when they're bagging up - as the chooks have discovered

What goes on my own veg beds and around fruit trees is likely to have no worms in it as they spread it for me and often demolish entire beds to make dustbaths after that, but the heaps are always wormy and they love it when scoops get removed

I don't fertilise my land (native ponies) but do have it sprayed 50% each year or more, for ragwort and other weeds so that chemical plus occasional ivermectin wormer, may be in the muck. When I have a big extra heap like I do this year (after a flood washed a lot of topsoil down where the ring feeders were so hay and slurry and mud mix scooped up and heaped to dry/rot) I am happy to respread it on my own land to replace the nutrition and as it is being grazed by the same stock then they've no extra chemical to deal with that they've not eaten already.
So for the sheep/hay acreage I'd be happy to use it personally, if I were you. And since I also use my own for my fruit trees and veg plots, without visible damage to the crops, I'd not worry about the rest either, but that's up to you. Unless you are registered organic and eat only organic you are probably taking in a lot more than whatever comes off horse manure. If you are registered organic, however, you possibly can't use the manure anyway. And I know a few farmers will take all kinds of straw, hemp etc bedding but not shavings due to the extra time/space requirements so perhaps that might be your threshold for deciding on the appropriateness of charging. You could also park a trailer down there for him to muck out into assuming you have a spare - again common on stable yards, so there is no additional work involved for you in collecting it, you just arrive with an empty and swap with the newly filled one.