From what you are telling me, the Postal Inspector was able to get a warrant based on the fact that drug sniffing dogs believed that there was contraband in the package. That gives them the probable cause to open the package.
From there, even if there are no drugs in the package, if the authorities believe -- as is typical when a person sends or receives large amounts of cash through the mail -- that money is the fruit or the instrumentality of a crime, they can seize that money under Federal forfeiture laws.
That said, they cannot take it away from you without due process of law. You have the right to a hearing on this and to try to show that the money came to you from a lawful source and for a lawful reason.
Whether you try to claim this money or not doesn't change the picture. The government will likely continue its investigation. You do have rights and they give you options: you can do nothing at all, in which case you'll forfeit the money. Or you can try to claim that money.
If you wish to do the latter, you'll be entitled to a hearing where you can be represented by counsel and attempt to demonstrate that this money has nothing to do with illegal activity. If you prevail, you'll get your money back. If you don't, the government gets to keep the money and anything you say in the proceedings can be used against you if indeed you end up prosecuted later on.