Is it usual for firms to claw back the cost of practising certificates?

Been with my firm for just over 5 years. Recently passed the SQE and qualified with them. I've g9t a new job and confirmed my leaving date but they're telling me that they will be taking back the cost of my practising certificate pro rata until the end of October? Is this normal?

I don't have a problem doing this but I've always met my billing target and have brought in alot of business over the years. Surely they can write off the cost of my practising certificate. Seems a bit like sour grapes. Is this common practice in the industry?