Ok I am not a lawyer so please do not take this as the gospel...but in similiar situations at least copyright the track.....dont listen to that other thread about you really dont have to because its automatically copywritten OMG LOL ......because especially if you have flipped a sample the odds are someone somewhere has that same exact sample flipped in some way that would resemble what you did by virtue of the same sample being implemented and at that point if they had copywritten it before you they could try and challenge your version and you would have no registered or certified copy to argue against it......if you found a sample that absolutey no one knows or thought of flippping you are in better shape but really how would you really know someone has not thought of it LOL....
It is the responsibility of the label or entity you sell that track to clear it, you need something in your contract if you are giving up exclusive rights....(I assume the bread you make up front is it??? no publishing??)
but yes you can sell that track just make sure you get your fair value on that track giving up the exclusive and especially if they dont want to give you publishing you have to get the most money out of it....but with them saying they are inking in a major deal I would somehow work in my publishing so I could at least get some royalties off of the spins this song might generate or points off the song if it goes on an album with some other producers...it looks like if you have something where they are going major and telling you that you need at least one of these to avoid being screwed...A really, really good manager and a really, really good lawyer in best case scenario....you have to remember you can control the usage since you are the copyright owner no matter if you sell the exclusive rights you need a contract stating so, I had a lawyer in Georgetown draft up my contracts and they look nothing like the internet contracts LOL.....there are contracts all over the net but IMO....I would get me a lawyer to customize my contract for how I want the beat used and also so there are no holes in the contract, you want an entertainment attorney to assist you and one that is familiar with music production and contract law pertaining to it.
if thats too much man just get some sort of terms hashed out on paper and have them sign and agree to your publishing and how you want them to use it....
My bottom line is this and you cant look at everyone as trustworthy in the game there are a lot of artists and labels out here to take your work as cheap as possible and then either take credit for it, sell thousands of mixtapes and then make you feel like you are overcharging or somehow wrong for trying to make profits off of your own work....be shrewd and crafty....get your money upfront and try to get your money in the end with a good contract and retention of publishing/credits/points or splits!