Assuming all purchases are made with a business credit card, and all cash back gets credited to the business income. There's three ways to do this, and one way is no better of worse than the other. You just pick what works best for you.
#1 - Reduce your expense by the amount of cash back received. This can be an accounting nightmare if you have a large number of purchases that qualify for the cash back incentive during the month.
#2 - Report the cash back as general business income.
Now others are screaming about #2 as "but wait! That makes it taxable income!" Yes, it absolutely does make it taxable income. The same exact amount of taxable income if you use #1 above.
#3 - List your credit card issuer (who you pay the bill to) as a vendor in your accounting software. When/if you pay the bill each month, it's recorded as paid to that vendor, including any interest for that month. If you need to separate out the interest payment on the credit card, that can be a sub-category for that vendor in your accounting software. If you get cash back, it's a refund from that vendor and is recorded as a refund in your accounting software.
For #3, if you're using Quickbooks 2018 or newer I can talk you through setting this up in QB if you need help with that. Just make a backup of your current QB data file before we/you go changing things.
