They can only do this with your consent, but you may have consented in the agreements covering your accounts or loans with the credit  union--that is, it's possible that the loan agreement, for example, says that the credit union may set off or apply against the amount owed to it by you any amounts it, or an affiliate of it (i.e. your employer), owes to you, such as severance pay. So you are correct in that you must have consented to this, and if they take the money without your consent, you could sue them for its return; however, you need to make sure  that you have not in fact signed anything which does give them this authority, and a good place to look are the loan agreements and other agreements governign your transactions with the credit union.