You seem uninterested in the details of how legislation is crafted to balance competing interests and values, and in that way both empower but also restrict regularity bodies. Congress did not say to Treasury "you can no 'limit financial transactions by people', good luck". If OFAC were to announce that they want to limit the number of bank transactions a person can do to 5 per day (say they are unable to investigate the larger volume), they would be exceeding their authority.

Another example, the tool that OFAC has here that we are talking about is their designation power, and the restrictions that follow such a designation are partly set in the statute defined by Congress, not something that OFAC has the power to make up.

> Then yes, OFAC would be able to prohibit this way of transferring money to Iran as well as any other way of doing it. Why not?

Transactions with the target entities ("Iran") are already restricted, whether through a crypto transfer or a tree-branch exchange. This is because OFAC has used its proper powers to designate those targets. The question at hand in this hypothetical is whether OFAC has the power /to designate the tree/ itself additionally, thus making it illegal for you to hang a sack of money there, regardless if you are transacting with Iran or just waiting for your cousin to pick it up.

Does OFAC have the power to say "we are designating the Hawala system"?