In the case of Commissioner of Internal Revenue v. Indianapolis Power & Light Co., 1989, the U.S. Supreme Court ruled in favor of Indianapolis Power & Light Co (IP&L). The issue at hand was whether customer deposits received by IP&L could be considered as taxable income or should be treated as loans that are not subject to tax under federal law. The IRS argued that these deposits were advance payments for services and hence constituted taxable income when received. However, IP&L contended they were security deposits acting more like loans from customers until their return or application to a final bill upon termination of service; thus, they shouldn't be taxed immediately upon receipt. The court held that since there was an obligation on part of IP&L to repay these amounts either by returning them directly or applying them against future bills and because no interest accrued on such funds while with the company, it did not have complete dominion over those funds - a key characteristic required for something to constitute income according to previous rulings (Glenshaw Glass standard). Therefore, these customer deposit amounts didn't qualify as gross income under section 61(a) of the Internal Revenue Code.

In the dissenting opinion of the case Commissioner of Internal Revenue v. Indianapolis Power & Light Co., Justice Blackmun argued that customer deposits should be considered as income for tax purposes at the time they are received, not when they're either applied to a final bill or returned to customers. He contended that these deposits were more than just loans because they provided an economic benefit to the company by reducing its need for working capital and borrowing. Furthermore, he pointed out that if a deposit is forfeited due to nonpayment, it becomes indistinguishable from any other form of income. Therefore, in his view, treating such deposits as current income would better reflect their true economic value and align with established principles of taxation law.