HGG Community Forums

The barns can be sold back under the "upgrades for HBs" section, there's a button to "sell barns", although since you get back less IVs than you bought them for I'd recommend keeping one or two extra small ones on hand in case you need them! You can't otherwise combine them, no.

As for pastures, you can't directly buy them with hbs, but you can pay hbs to another member to have them buy the pasture for you, just like the other posts you'll occasionally see around the forum. The rough exchange rate I think tends to be 150k hbs for every 1 USD of purchase price, but I've seen that number vary a bit both ways. All you have to do is save up enough, then make a forum post and offer " x hbs for x acre pasture" for whatever it is you're looking for.