A great explanation about this(m.youtube.com)
If Zelensky or the Ukrainian government enters into agreements with the U.S., the legitimacy and sustainability of these contracts may be questioned after the war, especially if they are perceived as self-serving or made without sufficient oversight or public consultation. A change in leadership post-conflict could also challenge the validity of such agreements.
The speaker of the parliament acts as president until a new one is elected, he could sign binding documents. But the "deal" is nothing more than a letter of intend that Zelensky or any other higher official could sign. A real treaty needs ratification by the parliaments of both countries.