If you get paid less, you pay taxes on less. The amount "donated in your name" is not actually donated by you, so it is not tax deductible by you on your tax return. It is not wages to you, it is not earned income to you. If there are any tax benefits to making the donation, they accrue to your employer.
[Edited to add]
While I believe my above response is technically correct, I also agree with @Critter-3 that, if the donations are only being made because it is part of compensation for your services, that is an illegal tax avoidance scheme.
There are legal ways for your employer to pay you compensation while reducing your taxable income. This mainly consists of tax deferred retirement plans, such as a 403B, 401(k), 457a or similar.
There are tax deferral schemes where you agree to defer a salary increase to a later date. However, if the plan is not “qualified” under IRS regulations, the deferred salary may still be taxable now. I am not well informed about non-qualified salary deferrals.
If you have already maximized your use of available qualified plans, you may have to simply refuse the raise or take the raise and eat the difference on your loan repayment scheme. After all, the loan repayment scheme is meant to help you in your early career when you are earning less. If your earnings are growing faster than expected, you don’t need the repayment scheme as much and, in fact, are using my tax money to repay your loan.
