they leased the beat you didn't sell your rights to the beat away and usually any producer's lease states that in the contract that's why you can lease the beat to a million artists,yes I said a million,trillion,quad-zillion artists
but once you sell exclusive you sell once,you can sell all rights to the beat or you can keep the rights for hopefully backend revenue from song/album sells,song played on tv shows/film/radio
sign up as a composer through BMI,ASCAP
and register your copyrights your instrumentals best to do them in bundle as(you have a copyright as soon as you make the beat)
you must register your copyright to protect yourself
U.S. Copyright Office - Online Services (eCO: Electronic Copyright Office)
U.S. Copyright Office - eCO Frequently Asked Questions
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