Hi I’d really appreciate some help and advice. I left my former employer in August of last year. My last day at work was the 1/8 after they told me to go home. There was some discussions about returning but for my mental health I couldn’t. My resignation was confirmed by them on 21/8. I was paid August salary (including some holiday owed) and now they want it back. I’ve been sent an invoice. I don’t have this money, what do I do?