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Legally, in Texas, unless they have a policy requiring that they pay out PTO at termination or resignation, they don't have to do so. Paying it even when you don't have a policy would create an established policy by practice, which is why the attorney told him not to pay.
Now, the fact that he told you he would pay is interesting, but for it to have any sort of binding effect, you'd have to prove that you behaved differently and to your detriment based on that statement. If you didn't give up something in return for that promise, it isn't contractual. You stated that you were resigning and had already planned to do that, not based on the promise of paid out PTO. That was just something he said after you gave notice.
So, unless you can show that you relied on that promise in such a way that you caused yourself harm from the reliance, there wouldn't be any sort of promissory estoppel claim. By reliance to your detriment, I mean more than simply that you really wanted it after being promised it and now that you don't get it, it's a hardship.
To give you an example, we normally see this claim when someone is offered a job but it isn't a contracted job, and before they start the job offer is rescinded. They can't sue for breach of contract because it wasn't contracted, but if they gave up other job opportunities in reasonable reliance on the promise of work, they can allege promissory estoppel. You haven't really stated what you gave up in return for that promise (so it isn't contractual) or how you actively were made worse off by your reliance on that promise. Without some argument like that, there isn't a legal argument to obtain that PTO.
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