If the amount of the bail exceeded $500. You must have the agreement in writing. Oral testimony (parole evidence) can be used if she can find someone who heard you acknowledge that you intended on paying her back. Unless you signed a promissory note or acknowledged that you would have to pay her back you are not liable. However, it would be the moral and proper thing to do since you likely would have been in jail for a long time but for her.
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I agree with Ms. Johnson, but really the facts could be tweaked slightly and you may be liable. For example, the fact that you are not together and so now she wants you to reimburse her indicates that perhaps originally it was clear that she posted bail as a gift. Having broken up, she wants to change the terms of the agreement. Absent some type of agreement from you to change the deal, I think she is probably out of luck, but...my sympathy is with her. So, tweak the facts slightly -- Your girlfriend may have a claim against you for something other than contract breach, such as unjust enrichment, reliance, etc. (I only bailed him out because he said he would get a job and pay the rent, but he didn't and then broke up with me and now I'm out $500). If she sued you she could recover her attorney fees if she won under a Washington statute for claims under $10,000.
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Another issue to know is that she could forfeit (lose) the bail money if you violate the terms of your bond, including the conditions of release set by the judge. Your liability to her depends on agreements you have regarding the money and whether it was a loan or a gift or simply a gesture of friendship with no strings attached.