Make sure you have this documented via email.
In writing, let them know that you ahve let them know that delivers the stems as they request, will in fact result in the stems not reflecting the final mix when summed.
And that a foreign deliverable will not be able to be made,begat meets specs, because of this being delivered teh way that is being requested.
This, so that when, not if, but when issues arise at a later date, you are not held accountable.
My experience has been, the minute you let the person know that you are shifting possiblefuture issues onto them, and documenting it, they change their tune.