Details like this need to be discussed whenever collaborations happen, so everyone knows the legalities.
If you've got a deal cooking with a Publisher or an Artist or the Artist's Management, the people whose signatures are necessary to make a Legal Contract should be 'in on' the negotiations, advised of terms of the deal, up front.
One 'holdout' can aggravate the people wanting the Song and give you a bad reputation if it interrupts the timing.
Usually they 'want' it now for a planned 'use' or release to market by a set time in their future. Slowing it down, or ending up unable to Sign the deal because of a holdout is bad for business.
If they have clear reasons for holding out and it is reasonable to try to negotiate to their satisfaction you can do that.
If it is simply naivete' about how the Music Biz operates, you may have to be the Educator.
Work on it. Make it happen. Get product to market. One good deal that gets to market gives all parties a credential for the next deal.

There will always be another song to be written. Someone will write it. Why not you? www.garyeandrews.com