There is a special rule for social security benefits in the first year you claim benefits. From SSA.gov:
"Sometimes people younger than full retirement age retire in the middle of the year and have already earned more than the yearly earnings limit. There is a special rule that applies to earnings for one year, usually the first year of retirement. Under this rule, you can get a full Social Security benefit for any whole month you are retired, regardless of your yearly earnings." (emphasis added). (https://faq.ssa.gov/link/portal/34011/34019/Article/3740/What-is-the-special-rule-about-earnings-in-the-first-year-of-retirement).
Since you claimed early, the wage limit of $15,720 for 2016 would normally preclude you from retaining any benefits, since you earned more than that amount. This transition rule allows you to be eligible for your December 21st check.
You can confirm this in two ways. The easiest is a visit to your SSA office to confirm the Dec 21 check is indeed on its way. The second is to enroll online for a SSA myAccount (https://www.ssa.gov/myaccount/) for a direct loink to the agency for account information, updates or questions.
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