This is poorly made-up conversation to find the loophole of Buddhism teachings especially no self idea (Anatman) but actually it does not prove any strong self (Atta) existence. The answers of Buddhist must be written by non-Buddhist who studied Buddha's teachings superficially and on this particular point "The Act of Will" called Chanda in Buddhism should be "I" and with strong belief that Buddhism cannot deny Chanda is not "I" or "mine".
The person who learned Buddha's teachings properly will reply Stoic questions differently. It could be verbally written like below.
Stoic: Yes, if you attach yourself to what is not given you will sure suffer. These five aggregates you counted must have been something not in your power. Have you nothing which is in your own power, which depends on yourself only and cannot be taken from you, or have you any thing of the kind?
The answer should be -
In Buddha's teachings there are only Sankhara Dhamma are occurring. There is none can be given to possess/control to be taken ownership. There is nothing a single person can possess (can tell this is mine, I am controlling it, I am taking it with me indefinitely, I have power to control/manage how it behaves...).
Stoic: what? Is any man able to make you assent to that which is false or compel you to desire what you do not wish?
The answer should be -
There is nothing can be called as "You" or "I" in here. In this Sankhara world where one person verbally say to another person to understand each other as we called people "Mr. or Mrs./Ms. A", "Mr. or Mrs./Ms. B", A can ask B to do something and vice versa. So called you ask so called me to do something and vice versa. This is the case. There is another case A does something on A's own desire/will/Chanda. This is the second case. Chanda is one of Nama Kaya and it is not permanent/eternal, not able to be possessed. It is one of Nama Kaya originated from Avidya (in plain English Ignorance). Because of this Ignorance a person viewed as Chanda/Will is mine, I did my Chanda to happen, I have Chanda therefore I am,...But in a period of single day, there are many countable obvious Chanda(s) happened and extinguished as well as many delicate Chanda(s) where a very good mindfulness and concentration require to notice them.
Stoic: In the matter of assent and desire then you are free from hindrance and obstruction?
This is something backing up the religions in which people believe there is god/creator/ruler (details below). In Buddha's teachings Avidya/Ignorance lead Citta Sinkhara (I memorize, I suffer) eventually lead to Chanda/Will/Desire. In proper Buddha's teachings we do not say my Chanda/Will/Desire. Laypeople (one who didn't get any attainment to be enlightened) with Atta Samjna(the perception of existence of "Self" which has either eternal or being controllable/possess-able or both) see the mind is "Self" (I think therefore I am/I desire therefore I am). But in Buddha's teachings there is no such entity callable "Self".
The idea of "Act of will"/Will/Free Will is originated from religions of God(s). For the early believers of God, everything is under God's will. But modern people found that is dangerous and outlaw idea. If someone burn other people or bomb a group of people to death/injured and that person says it is God's will then religion of such people does not help very much about the damage in both physical and ethical. So new civilized people invented a things called Free Will or Will of people which is separated from God's will. By doing so God is relieved from inappropriate responsibility of a single person wrong doing as well as ethical and legal constraint can posted and it is publicly and religiously acceptable. It is obvious that Physical Bodies (Rupa Khandha) is Anatta (not Atta) for Westerners (Middle easterners and Mediterraneans of early Buddhism ages) but they do not fully understand other four Khandhas (Vedana, Samjna, Sankhara, Vijnana Khandas) or called soul to one of them. So all four Khandas combined to a word called mind or soul and they thought there is a chance to find a loophole of Anatta (no self, no Atta) of Buddha's teachings because they failed to notice the latches and couplers between two consecutive transitions of state of mind since Citta Khandas very short-lived and very fast. Only the trained person to do Vipasana with strong concentration can see the details transitions of Nama Kaya.
There are many anatman definitions and explanations in Buddhism literature both printed and web but here is concise repetition here.
I see, therefore I am.
No, I see because I have eyes and things to views. When these two contact I have a sequence of thinking, memorizing, examining and judging occurs in my mind. Seeing doesn't prove it is me
I hear, therefore I am.
...As above seeing
I smell, therefore I am
I can taste, therefore I am
I can feel the touch, therefore I am
...all as above seeing
I think, therefore I am
the contact of things memorized and mind makes thinking, judging and examining mentally and all phases are temporary, not controllable, not possess-able and extinguished/depleted eventually. In every steps and phases of transitions of state of mind (or called soul) a properly taught Buddhist cannot find Atta Samjna (the idea of perception of self). That is true, all mental bodies (four Khandas, Vedana/suffer, Samjna/memorize/percept, Sankhara/initiate/encourage/activate, Vijnana/be conscious or called soul to one of four) are Anatta (not to be possessed, have an end, can deplete, can extinguish). Chanda is also Anatta in both wholesome desire/Chanda and unwholesome desire/Chanda.
Buddha said when a person is doing meditation or Vipasana only Samkhara Khanda is observing or insightfully viewing to other Samkhara Khanda. That means we cannot find I or Me or My or Mine even in doing meditation/Vipasana. Verbally we say I meditate, I do Vipasana but it is just for the sake of language and colloquial sayings. There is no such case that you are going to find any sixth Khanda or/and a Khanda that can be called "I", "Mine" or "Self" in Buddha's teaching or outside Buddha's teaching. This also applied to both external and internal from a viewpoint of a person's Khandas. So will/volition/Chanda or whatever Stoic called cannot be "I" or "Mine" because it has originated cause "Ignorance/Avidya" and other supporting factors which are not controllable/possess-able by a person. In conclusion, any Stoic or early/late philosopher cannot prove a single entity (in physical or metal form) that can be self (either can control/possess, persist, eternal, can be created, can be managed).