Final answer:
The internal control principle that requires providing a sales receipt to customers is 'Document procedures'. This principle ensures that transactions are properly recorded and helps maintain the integrity of sales records.
Step-by-step explanation:
The internal control principle that best demonstrates the requirement that all customers be given a sales receipt is Document procedures. This principle ensures that proper records are maintained for all transactions. Providing customers with sales receipts is a part of this as it serves as a proof of sale and helps in tracking sales transactions, thereby reinforcing the accuracy and integrity of sales records. Each receipt represents documentation that a particular sale occurred, detailing the goods or services provided, the amount paid, and other relevant transaction information.