Raisin sprinkles will shake out as you make the hay and if one has had the runs then you would have to remove the contaminated bit by hand. The sheep, if left in the field, may well sleep on the hay you are making, eat it and as you say, pee on it - the finished product won't be tasty for them. We shut up our field destined for hay as soon as the grass starts growing in spring, then we cut it when it's ready and we have a haymaking weather window. Takes about 10 weeks. We wouldn't leave the sheep on it at all during that time, for the reasons above.
However, if you are just cutting grassy corners, then I would think that as long as the sheep are out once you are ready to cut that will be ok. If they are contaminating it that means they have been eating it and sleeping on it, so there won't be anything left worth scything. In fact I'm wondering why the corners are not being grazed if there are normally sheep on it - not worth the effort of making hay if it's stuff they won't eat.