Employers who wish to claim that a payment is a loan and not a sign-on bonus need to be clear and consistent. Here are suggestions to consider:
Do not have a signing bonus policy. Do not use this terminology for any reason. A payment called a "signing bonus" means the amounts are wages.
Have a formal, published employee loan policy directed at new hires. The policy should be clear on the scope of the forgiveness rules.
Have a formal loan agreement that follows state law. This could range from a simple one page agreement to a document prepared by a lawyer.
Have a separate formal voluntary deduction agreement signed by the employee prepared before any payment is made if there is any chance that payroll will need to take a deduction. This is required under the laws of some states.
Payroll needs a copy of the loan agreement and the voluntary deduction agreement at the time the payment is made. Payroll needs to be compliant with the IRS rule on recognizing the wage payment as soon as the forgiveness occurs. Failure to comply can be used by IRS as evidence that the payments were not loans.
Loans in excess of $10,000 cause interest to occur. If the employee does not pay the interest, then that must be accounted for by payroll.