Agree with PP. in the better places I’ve worked, there would be an After Action investigation to understand why procedures and review processes failed to pick up the problem.
if it was found you acted without due care or integrity (eg you assured your manager you’d checked and followed up and had done all the steps, whereas in fact you hadn’t) yes it would be a problem
if it was found that you followed the proper process but there was poor training, an ineffective procedure for managing the reclaims, reconciling the amounts expected, and ineffective supervision etc then it would be handled differently
if you are generally known to be competent and careful then this would feed into the review also.
in reality what does sacking you achieve? The money is gone; then they just train someone else badly and the problem recurs.
Mistakes happen; finance management is all about managing the risk of human or system error and how much is the organisation willing to invest in managing the risk.
Im sure you feel sick to the stomach about it, but hold tight. Be honest, be willing to accept where you need to learn and improve, and be constructive pointing out how the error could be avoided in future. Good luck.