Can I change the closing line on my receipts for Ramadan and put it back afterwards?

Yes, whenever you like. Open Settings → Printing & receipts and edit A closing line of your own under What the receipt says, then press Save. The panel beside the form shows exactly what will print, built by the same code that prints the paper. Every device picks up the new line within a minute, and putting the old line back is the same edit. The POS and the counter tab in the docs covers what the receipt says.