The phrase "to pocket the difference" means to keep the extra money or profit that remains after deducting expenses or comparing different prices. Full definition
You can then use the new loan to pay off your old loan and pocket the difference as cash to be used for whatever you need. (badcredit.org)
In this situation, you would be responsible to pay out of pocket the difference between the brand new car and the depreciated value of the rental car. (encharter.com)
The company could tell the IRS that you make much less money than you do, and pocket the difference in tax. (politics.stackexchange.com)