Pony up is slang, and has connotations of unpleasantness. If you belong to an organization, a condominium for example, that unexpectedly requires you to make additional financial contributions, you have to pony up. If your stockbroker gives you a margin call, you have to pony up. And if the mob is extorting money from you, again you pony up. I would never expect to hear pony up in an ordinary business context such as your bank example.