just look up what a lease is. there are leased cars too.
including a clause in a contract that the owning party can cancel the lease any time would really drive down the price of the lease. so i would presume no there is no such clause. and airlines would never agree to such a clause.
so if the engine is currently leased the manufacturer wouldn't have the right to take possession of the engine even if it was a spare.
what the other poster meant: emirates leases quite a few engines so it's logical to assume EA would have an engine ready to lease for emirates or one for that the lease has expired.