Comlink-x: they sold Yosef because they thought it was the right thing to do. They made a mistake in that, as we know from Chazal. Once they sold him (thinking it was the right thing to do), they lied about it. Chazal tell us that that they made a mistake in selling Yosef. I am not aware that they tell us that the Shevatim made a mistake in lying about it (It could be that they do, but you would need to find a source). I think it’s unlikely that they would have made a mistake about the halacha regarding lying in such a circumstance. This is a separate issue from Mechiras Yosef. There are two issues here:
1. The selling of Yosef
2. If selling Yosef was the right thing to do (as they assumed), would lying about it be okay?
Basically, whether or not selling Yosef was really the right thing to do in the first place, their psak halacha about lying had to do with lying when you did something RIGHT (since they had thought they did something right). So even if it was wrong, their Psak Halacha was based on what one should do when he has done the right thing, so we can learn from it regarding whether or not one should lie when he HAS acted correctly.