When customers return products, the billing process typically involves adjusting the original invoice or issuing a credit note. If only a portion of a product has been used before its return, the billing would reflect this usage. The amount credited back to the customer would be reduced to account for the consumed value.

For example, if a customer purchased a consumable item and used 50% of it before returning, the seller might calculate a prorated refund based on the percentage of use. The refunded amount would be less than the full purchase price, reflecting the value of the used portion.

In accounting terms, this would involve crediting the sales revenue account for the returned amount and potentially debiting a cost of goods sold (COGS) account for the consumed portion, assuming the product cannot be resold as new. The exact accounting treatment would depend on the specific circumstances and the company's policies.

However, the details provided don't specify a particular accounting standard or situation, so the analysis is based on general business practices. For a more precise answer, additional information about the accounting policies, the nature of the product, and the applicable laws or standards would be needed.