As someone has already posted 99% of aircraft can't descend and slow down at the same time. What I've always taught and used I will issue one of two instructions:
Reduce speed to XXX then descend and maintain XXX.
Desecend and maintain XXX, then reduce speed to XXX.
If I need an aircraft to level off and reduce speed then start descending again I will issue an appropriate instruction.
If I'm handed off an aircraft that is restricted I will re-evaluate their restriction and either stick with it or issue a new instruction once they enter my airspace.