Vick2005 Roo

Hi. In 2018/19, my employer mistakenly deducted more pension contributions from my salary than they should have. They told me this later in 2019. I realised they hadn’t repaid them last year & asked them to do so. They paid me half the amount owing last November & will pay the rest later this month (April 24). I do SA tax returns. Should I declare this income in my 23/24 & 24/25 tax returns? If not & I declare it in 18/19, how do I do this, now it’s 5 years later? It matters because I was paying basic rate tax then, but have now crossed the higher rate threshold. Thus, if I declare the income in the year it was earned (18/19) I’ll pay 20% tax, but if I declare it in 23/24 & 24/25 I’ll pay 40% tax.