You obviously knew that he couldn’t guarantee anything that was outside his control, and it was on that basis that you gave the money.
The gemara in Rosh Hashona says explicitly that the reason it is permitted to give tzedakah in order to achieve a specific outcome is that a yid understands that Hashem always does what’s best, and if he doesn’t get the outcome he wanted he doesn’t complain and demand his money back. Whereas a nochri doesn’t understand that, and thinks that the tzedoka is a transaction, consideration for services to be rendered, and if Hashem doesn’t render those services he feels cheated. And therefore the gemara says a nochri is not allowed to make such a donation, while a yid is.
I understand that in this case your complaint is not against Hashem but against the person who you allege made “promises”, but since you knew in advance that he doesn’t control Hashem, it amounts to the same thing, and you wanting your money back seems like the behavior that the gemara ascribes to nochrim.