So, if I’m understanding this correctly (which, given it’s a Friday evening, is very unlikely 
), I don’t think you’ll benefit here…
This is from the T’s and C’s
Examples
When you won’t pay interest on purchases
If you paid the full amount you owe in June and in July we won’t charge interest on any purchase charged to your account in July and shown in your July statement.
When you’ll pay interest on purchases
If you didn’t pay the full amount you owe in June, we’ll charge interest on any purchase charged to your account in June and July, even if you pay the full amount you owe in July.
So, in my head, unless you pay off the sofa the month after you buy it, you’ll end up paying interest on it.
The best thing to do would be to take out the card, but the sofa, pay the card off with the cash, enjoy the cashback…
I think that makes sense?