I wouldn't offer someone money at a checkout tbh because of a recent experience I had.
I was in Tesco with the dc a few weeks back. We'd got a few bits which came to about £30. Got to the checkout, pulled my debit card out and realised it was for the joint account, which I was pretty certain was empty (we only use it for bills and it's empty-ish for 2/3 of the month). My sole account had plenty in...it was payday! I didn't have my phone on me so couldn't even call the bank and do a quick transfer from my account to the joint account.
As I realised, the cashier was reaching for my card, so I handed it over, crossing my fingers that there was more in there than I thought. The bloody thing declined.
I'm shit in these situations, so blushed beetroot, apologised and said I'd brought the wrong card, my money was in my other account and I'd have to leave it...it sounded completely 'the lady doth protest too much' and I could practically feel the sympathetic eyes of the queue of people, and the cashier 
As if it couldn't get any worse, a man behind me said 'hey...let me' with a kind smile, holding out his credit card. I stuttered and said thank you, but there was really no need, I just needed to nip home to get money and would come back.
He insisted, I again politely said there was really no need and he said 'no please let me...I don't want to see kiddies go without'

What the fuck do you do? If I'd turned and left it would have made him feel awful, and the queue was getting pissed off. I let him pay.
I know he meant it kindly and he obviously thought I was skint but, really, it's probably one of the most humiliating things to have happened to me.