[disclaimer level: haven't played adventure mode in a few weeks, don't remember what the screens look like, and don't have a df install handy at the moment]
your currency needs to match the civ of the person you're trading with, which is very often different from the city or civ they're nominally in, which is a big part of what makes the whole coins-as-currency such a crapshoot. they're also often carrying coins they don't even recognise themselves...
if you can see the coins they're carrying in the left side of the trade screen, along with all their other objects, that means their own coins are in a currency they don't recognise(!). they'll happily trade these to you as normal objects with a value of 1, regardless of their face value.... if you don't see coins-as-ordinary-objects in their trade list, that means that whatever coins they're carrying are coins they recognise as currency. in which case you can see what coins they're carrying by [l]ooking at them outside the trade/exchange goods window to learn what currency they accept
but, when you use the [a]sk/[ o]ffer currency keys it will automatically use the correct currency, if you have a mutually agreeable one. if you don't have a mutually agreeable one, the maximum value you can ask or offer will be zero; if you do have a mutually agreeable currency, then the maximums will be your respective holdings in that currency. you'll need to type in how much you want to ask or offer, it's not automatic... if you didn't type in an amount to offer, then you don't have the right currency and haven't actually ask/offered anything