Negotiate. Keep lowering the wages, the fees etc until you can afford it (i.e. it will no longer be red). In the boardroom, have you moved the bar all the way to the left as someone suggested earlier? If so, don't do that for now and move it back until you have enough spare wage budget to meet Munoz's demands.
The best thing to do here is change his squad status to Rotation, allowing you to offer more. Offer him around 20-25k per week, bump the agent's fee up a bit more and remove the 15% yearly wage increase clause (you never want those). You can generally lower everything but increase the agent fee and they'll sign.
You can always change his status back to backup/hot prospect after he's signed. He'll probably be unhappy, but unlike FM10 you can now talk to him and say "you didn't deserve the previous squad status".