This may be a dumb question, but can my employer make my IBR payments for me? 

Here’s my scenario: my employer has offered me a substantial raise.  However, if I take the raise, my IBR monthly payment will increase.  Instead, I could keep my current salary but have my employer make my IBR payments for me.  The way the math works out, I’d end up with more money in my pocket this way.  Is this legal?  Thanks in advance!

From a student loan perspective, it doesn’t matter who makes your student loan payment, including IBR payments.  From a tax perspective, though, your employer may be obligated to report it to the IRS as a fringe benefit on your W-2, thereby removing the benefit you were seeking of keeping your income low for IBR purposes.  Either way, it’s on them to determine whether it’s something that they need to report.

Your employer can give you loan repayment assistance benefits, sure, but they are taxable income to you and so have the same effect on your AGI and IBR payments as a raise.