There are two ways this could happen, either of them have the same effect for the company handling the donation. First, McDonald's takes the money and sets it

There are two ways this could happen, either of them have the same effect for the company handling the donation. First, McDonald's takes the money and sets it aside, using their POS systems to segregate these funds from their traditional revenue. At the end of whatever time frame, they send the accumulated funds to the non-profit. The consumer can use these donations as a write-off by simply keeping their itemized receipt and showing the round-up. Second, McDonald's could collect the funds as revenue, then make the donation as themselves. This is most likely not legal, since the consumer can also count the revenue as charitable donations, so it wouldn't make sense to have the same funds sent for the same purpose but tallied as a donation from two entities. However, this money would be revenue, then a donation, so it would have an impact against their legitimate taxable revenue/profits. For example, let's say McDonald's earns $1b in revenue, including the consumers' donations of $10m and amount to a total profit of $100m. Their profits without the donations is $90m. By donating the $10m from their $100m in profits, their lowering their taxable profits to $90m, making for the same tax obligation as if they didn't take the money in as income. The only place for nefarious activity in all this would be the potential for earning interest from the donations in the time between receiving the funds and forwarding them to the charity. Curious how they would be able to justify those earnings, if they did something like that.