Why would a customer ask for a receipt date...

[Freelancier]

Ok, now that you're clearer, yes, a credit card transaction receipt is a legal document in the eyes of the credit card company. You don't mess with that. But it's less of a problem to manipulate your own receipt as long as the date of payment is the same as the matching credit card receipt. In other words, I order something online, the date of the order is one date, the date of charge is another, and the date of shipping is yet another (which should also be the date when the charge is put in, but sometimes it's not), and the date I submit the expense to my company is yet another... so which is the "date" as it pertains to the expense? From my company's perspective, it's usually the date I submit it for reimbursement, not the date it incurred. From my personal perspective, it's the date I get the credit card statement (or maybe the date of shipment, since that often starts the clock for returns).

So you can see there's a bit of flexibility in the system as it relates to the date of a transaction.

Anyway, you solved the issue in your way and congrats for doing that.

[Adel Landman Steyn]

My first guess is tax fraud. If it was the other way around, I would be guessing liquidity issues.
The thing is, they want to make their expenses more for the previous financial year. Accounting and tax for the most part, works on the principle that the creditor was established on the date of delivery.
Even though they are using a "credit" card to pay, the obligation to you did not necessarily arise on the same day as the date on their credit card statement. The date on their credit card statement, is only the date that they transferred your obligation to you by creating a new obligation with their credit card company. The person could have paid you with a "debit" card or EFT as well. This does not mean that you did not sell the goods on terms.
The question now becomes, do you even offer terms to any of your customers? If you deliver strictly COD, then yes, if it was picked up in a sample when their financials are audited, that could probably lead to a few questions.
I don't recommend changing the date of actual delivery to something it is not, but this is probably something they should be more worried about than you...