No.

You enter into an agreement to repay the money, if you don't fulfill your part. Period. End of story. The tax that you pay isn't leverage. It's your problem. You owe tax, you pay it.

The airline offers you a certain amount of money. That's what comes out of their coffers. What you do with that money is your choice. Also your problem. If you don't fulfill your end, and you have to pay it back, then you have to pay it all back. They gave it to you. You have to give it back.

Call it incentive, leverage, pressure, enticement, a carrot, or whatever you will; it's the airline sweetening the deal to get you to come to work. This sweetness comes with terms, as do all contracts. It behooves anyone entering into an agreement to know the terms. You don't fly an airplane without knowing the limitations; exceed them, and there may be a price to be paid. You don't enter into a contract with the airline without knowing the terms. Violate them, and it may cost you; there is a price to be paid.

Yes, the idea is that by taking the money, you will be honorable and fulfill your end of the bargain. If you don't, then yes, it will cost you. That's the whole point of the agreement. It's not a free lunch.

The tax is a requirement of the federal government. That you are taxed on your income does not change the fact that the airline offered you the conditional bonus; if you owe the conditional bonus back for failing to abide the conditions, the airline wants its money back. All of it. Regardless of what you did with the money. Regardless of whether you gave it to the government, your ex-wife, or bought a really nice enclosure for your Goldfish, Sally.

This leaves you with choice: stay and fulfill your obligation with time in service, or pay back the money you owe for failing to fulfill your obligation. The company hopes you do the former. That's why they offered the incentive. If you choose the latter, they expect you to honor that. Otherwise, there may be legal action.

For a long time now, bad advice has circulated, "those contracts aren't enforceable." Listen to bad advice at your own risk.