I can't quite see what the answer above has to do with the phrase 'in salt' in the question, as opposed to 'below the salt' . Generally speaking, 'in salt' means in brine or pickled. Usually, the connection between 'pickled' and 'drinks' is that you have had too many of them!
I have never heard the phrase used, despite decades of pub visiting, but I'd imagine - if a pub andlord says "you have drinks in salt" - that would suggest that they have already been paid for...ie they are (p)reserved... perhaps by friends, and you are entitled to them at no further cost to yourself.