The Internal Revenue Service has no problem with you requesting a tax deduction for cash donations to favorite causes. It does, however, want more than your word as proof of your generosity -- it wants receipts. If you follow the receipt rules, both you and the charity benefit from your actions. Do it wrong and the charity will get to keep the cash, but it won’t be any help to you at tax time.
Bank records are always acceptable documentation for cash contributions. It can be a credit card receipt, a canceled check, or a whole bank statement as long as it includes the date of the contribution, the organization’s name, and the amount. A receipt or letter from the charity is OK if it contains the same information. So are pay records if they show a deduction made to the charity, and it's backed up by something like a pledge card.
You need extra proof when you make a single large donation of $250 or more. A bunch of small donations, such as $10 a week to your church or the local animal shelter, don’t count here. The IRS needs written acknowledgement from the charity for any large donation, whether you use cash, check or credit card. If it’s a payroll deduction, your pay stub has to show when you made the donation, what you made it to, and for how much. You’ll still need a pledge card or something similar from the charity.
The IRS only accepts a donation acknowledgement if it meets their very specific requirements. It can be a letter or a receipt, but it must clearly be from the charity, it must be in writing, and you have to get it at the time you make the donation. The IRS also wants to see the date of the donation, the exact amount, and if you got anything out of it, such as a dinner or tickets to a ball game. They’ll deduct the fair market value of any benefit to come up with the amount you can actually deduct.
Even if you have a receipt, it won't count if the charity is not a "qualified" group. Take a look at the government’s Select Check site to see if the IRS considers your cause qualified. Also, the IRS puts limits on how much of your income you can donate each year. Like everything else they do, there’s plenty of qualifications, rules, and exceptions that apply to those limits. In general, you can’t give away more than half of your adjusted gross income and expect to deduct it.