Budgets are set at the campaign level and are easily changed at any time. A daily budget is decided according to the amount you are comfortable spending each day and is based on your business and advertising goals as well as your expected return on investment for the campaign.
It’s important to know that up to 2 times your campaign’s daily budget can be used to show Ads in one day. However, when paying for conversions your daily costs may be more than 2 times. Costs may exceed your daily budget, however, in a calendar month, costs will never exceed your daily budget multiplied by the average number of days in a month (30.4). Google Ads does this in order to give your Ads the best chance of receiving impressions and clicks so that your campaign is maximizing your return on investment by showing your Ads more often when search traffic is higher.
In the event that your daily costs exceed 2 times your daily budget, you will only be charged the 2 times over your daily budget – anything that goes over your 2 times daily budget amount will be returned to you as an overdelivery credit.
If you are charged more than your monthly charging limit, that is, your daily budget multiplied by 30.4—the amount overcharged will be returned to you as an overdelivery credit.
When changing your budget, your monthly costs will still not exceed that of your new daily budget multiplied by the number of days left for that month.
Source: About account budgets