What can i do about my boss matching my social security with my own money?

my boss is writing me a personal check and matching my social security with my own money

I am not clear on the question, but some of the answers are definitely incorrect. Social Security and Medicare (OASDI/HI) funding by law requires that everyone must pay a certain percentage of their compensation. There are two categories: employed or self employed /consultant /independent contractor. Employees have a certain percentage deducted from their pay, and the employer must pay in an equal amount. Self employed must pay a certain percentage, more than an employee but less than the sum of the employee and employer.
You can describe the employee situation as "your money" in the same sense that withheld tax is "your money". Some economists say it is not your money because it is a payroll tax that disappears before you ever see it. Other economists say that both the employee portion and the employer portion are "your money" because the employer would presumable pay the employee more if he didn't have to pay the OASDI to the government. No matter what you call it, though, both an employee and an employer must contribute to OASDI/HI

Sorry, I'm unclear on your question

Could you please be more specific?

I think you're confusing Social Security with a 401k/pension plan.
Your employer is legally required to direct part of your paycheck to Social Security, just as part of your paycheck is automatically deducted for federal and state taxes. Yes, it is your own money - the government is forcing you to be prudent and save ahead for your own retirement.

The employer is not required to "match" your Social Security payments or pay anything at all to your Social Security account. However, many employers offer a similar retirement savings plan, called a 401k or pension plan. Here, the employer will match your donations up to a certain amount. This is "free money" and you should definitely take it if offered.

The bottom line is that some percentage must be paid.
Isn't it about 12.5% now. Where the money comes from I don't think is set in stone.

If you are self employed you get to pay it all. Its just a given in most cases that the employer will match. Is it the law? I don't think so. Remember you and your employer have a contract. The terms of the contract are up to you and your employer. You could have a setup where the employer pays all of the ssi I would imagine.

However I don't know the law. If you force the employer to pay your match, will he just lower your pay to make up the difference?