We obviously don't know what was in the contract. But it's possible the option clause was based on the assumption that NYC might want to exercise it to avoid negotiating against Boca or any other team in the world. Had we exercised, we would own Bravo's contract no question. When we failed to exercise he became a free agent, and could go back to Boca, or to any other suitor. If we were willing to risk that in an effort to get him without the option fee, we should be allowed to do so. it gets murkier if we colluded with Bravo to make a side deal get out of the fee whereby we agreed on a contract before the exercise date, but did not execute it, and then do so only when we can do so for no fee. But even there the side deal would have to be something either party could renege upon in order to properly evade the option fee -- which means there is still risk -- which means it is (under this scenario) still OK. Less defensible would be an enforceable non-soccer contract that gets converted to a soccer contract later on, sort of a variant on the Lampard scenario. I have to figure that is not allowed.
None of the above takes into account whether there are specific FIFA rules, or a standard industry understanding based on pattern and practice, that forbid these sorts of tactics. And if the loan deal between NYC and Boca proscribes these then of course they are out of the question. For example, the loan deal could provide that NYC cannot sign Bravo for __ months after the loan ends unless they exercise the option and pay the fee.