Assalamualaikum ...
"How would you like your money sir?"
If she(the lady at the counter) was about to receive the payment from the customer, she could as well mean the form of the payment which could be cash or credit card. This is similar to "Would you like to pay cash or with credit card sir?". So the answer should be "I will pay cash" or "I will pay with credit card".
If she has received the payment and was about to return some balance (or whatever), she most likely meant the format of the money which could be the notes/bills of RM10, RM50, RM100 or so on. This is similar to "What note would you like the balance in sir ?" or maybe a little specific like "Would you like your balance in 10s or 50s sir?" .... So the answer should be "I would like it in 10s and 50s please" or whatever notes one wants.
Im sure we (me,you, the people) have encountered sentences like ...
"How would you like your coffee sir/maam?" which can be answered with
"I would like my coffee hot/cold/iced/sweet/strong please." There is also
"Would you like to join me in a cup of coffee?" which can be answered with
"Yes i would love to. Thank you" or you can be devilishly playful and answer
"Yes sure. You jump in first".
This type of vague* question is quite common in the world of hospitality (hotel, services, etc). It is a level of politeness (hehh!!). In fact, it is quite an old form of being polite, and yet classique.
I always find this particular sentence hilarious and i tend to have evil thought in mind whenever a person says this sentence to me .. "If you walk this way please". It means, "Please follow me". But literally, to walk his or her way can also be disastrous
if yall know what i mean ... Kalau dia melenggang kangkung, kita pun kena melenggang jugaklah gamaknye.
However, not knowing the detail of the situation that Fibix posed up there, i cant say for sure what did the girl and the boy mean by the question because people tend to misuse this form of query these days.Anyway, here is a little hoot-hoot to check out ...
1.
How do you find Malaysia ?Answer A: I find it (to be) very beautiful and green.
Answer B: I bought a map and there it was; Malaysia on page 23.
2.
Do you serve crabs here ?Answer A: Yes we do; delicious crabs with cheese sauce.
Answer B: Oh just sit down , we serve everybody here; no discrimination.
In Jamaica, if one is asked "How would you like your money sir", you should always answer "Perfect please." This means, "Keep the change", that is, if your balance is $56.85, you will pay $60. If $78, you will pay $80. But if your balance is less than $10, you
should always answer "No thank you" which means "Keep it all". I said 'should always' because it is the culture of hospitality/service there.
Ok eloklah saya berenti menaip ....Nanti berjela jela pulak ....
Cik Bedah
Temerloh
p/s: these days, people say something to mean something else ...
*vague = not precise, not direct
: How do you like your money sir?
honestly, i never heard this words before... at that time i'm were about to ask what her mean by that, or just answer it on how i like to spend my money... 
... i have this disease since i was about 7 years old...It is called Lengthereplisitis ...
