Cancelling an "Open" trade

I need a little help/ advice on cancelling an open trade. I replied to an ISO that listed a specific barrel aged variant. During the negotiations, I specifically asked if his beer listed was indeed the specified variant by asking "which variant is this"? He responded positively that it was said variant. We agreed upon a trade, and I promptly shipped out (gotta get the hops there fresh). After 2 days with no shipping information, I sent him a message inquiring to the status of his shipment. He responded, and told me he would ship the following day. Subsequently, he sent me a message to me stating that he did NOT have said variant (it was the regular version). I asked him to ship my stuff back when he got it, to which he has agreed. My question is, how do I cancel this trade? If I cannot cancel, what is the correct course of action to take? Any help would be GREATLY appreciated. Thanks in advance.

 

Thanks, Dave!