"Cave" are only allowed to do a fix amount of botttles a year so when production is good they have to much wine product so they can bottle them but not sell them as bottle as they dont have the tax stamp. It could happen sometimes that its sold in black market and not to show unallowed bottle can be served by the glass....... But of course this is purely theorical as everyone respect the law....

You should definitely put out the context of this sentence. Is it really a joke or a funny way to look at things? “Surplus de cave” refers to an overproduction of wine. “Le surplus” is the quantity in excess. Now, wine should be kept in bottles or barrels to get older and taste better when it comes to the quality. Nonetheless, you can drink it even when it is still “young” as we say in France. So, I guess that “à passer au verre” could mean that the quantity in excess is given in order to drink immediately. I hope that makes sense.