A cashback is now usually used as a marketing device, as the comments show. Earlier, at least in the UK and before cash machines worked well, supermarkets promoted it to allow customers to take cash from their own bank account when they paid using a debit card. The customer might then spend that cash in the store. Also, at that time a customer might need come to the store and buy goods mainly to obtain the cash.
When it is a single word, I think it's also a noun: compare 'get $30 change', 'get $30 interest', 'get $30 compensation'. Switching to the numeral 'one', we can see the sum is in plain case: 'get one dollar interest' (not *'one dollar's').
In these, does 'get' take one noun phrase complement or two? Compare: '$30 interest would be nice', 'You owe me $30 change' (sequence of three NP complements otherwise unexampled), 'I'm waiting for my $30 refund'. The sequence moves around tightly like a single NP. Also: you can't replace the sum by a pronoun. If $30 is the topic, you can't say you got it refund, or got it interest. From all which I conclude '$30 cashback' is a single noun phrase composed of two noun parts (nominals or N') in apposition.
I like compound words and I've always liked cashback. The double vowel-sound calls to mind a wad of notes being riffled and slapped down on the counter, or a till clicking open - it has a satisfying transactional ring to it.
Yes, I've seen "cashback" used as a noun, mostly regarding car sales. But in most instances, even in car-sale ads, it seems to be used not as a noun, which would seem to mean it should be two separate words. "Get back $3,000 cash" means the same as "Get $3,000 cash back," and "Get $3,000 cashback" just ain't fittin'.
And, it's a stupid gimmick anyway, because if you're getting "cash back," then you shouldn't be paying interest on it with your car loan, should you? *hates car salesmen*
Anyway, FWIW, and I realize nobody probably cares, I strongly prefer two words over one in this instance.
I'm not familiar with the meaning qroqqa has in mind. It's cash out in Australia, and to be honest I've never paused to consider whether it might be cashout.
We do use cashback in the second sense as below. You might see a video camera advertised on sale for '$600 with $80 cashback'. At the retailer you must pay the $600, but they will give you some kind of voucher which you then must send to the manufacturer/distributor, who eventually coughs up the 80 quid.
Usually one word, it seems to me, even when two would make equal sense: 'Would you like cashback?', 'Could I have £20 cashback, please?' And what is the difference between one word and two? Secondary stress, I think: in 'get £20 cash back', the 'back' has a longer vowel, as far as I can tell. (Possibly there is even a slight effect on the 'cash' vowel.)
Googling for this, I found it has a second meaning, which I've never encountered (as far as I'm aware): cash offered back as a sales incentive. Both meanings are in the OED, the familiar EFTPOS sense later (1988) and marked 'chiefly Brit.'
Googling for this again, two words is more common in casual/Web writing, but I think this ignores the phonetic facts.