I don't think bud is "legal" in any part of Latin America but in general it is much more decriminalized than in the U.S. They don't lock away tourists for buy or having a little for personal use. Selling and trafficing those carry serious consiquences. I lived in several places in South and Central America. I never heard of a tourist getting locked up for a little bit of personal. The best way to find out about the local laws, is from the local people once you are there. Keep your wits about you, and you will have to make a dissicion about how you feel buying it from someone. Use your better judgment but dont be paranoid and you will be fine.
Honestly, the worst that could happen is that someone sets you up for a little "sting" opereration with a local cop. Because they know there will be a lot of paranoid pot-head tourists that dont know shit about local law. So the cop will then proceed to ask you for the amount of $ that he thinks you will pay. this is normally 20-100$ Then the cop and the guy you bought the drugs from will split the cash. This is actually a comon little setup in highly touristy areas in Latin America. This has happened to a few people I know. If you are not going to be in a very touristy area, and I don't think Nicaragua is very touristy, that you don't have much to worry about. I have traveled all over latin america, and I have bought lots of pot everywere, no problems. Like I said definately do NOT be paranoid, enjoy yourself, but don't be a complete dumb ass either
Cheers