29 CFR §100.612

1. (a)A debtor who disputes a debt should provide the NLRB with an explanation as to why the debt is incorrect within 60 days from the date the initial demand letter was mailed or hand-delivered. The debtor may support the explanation by affidavits, cancelled checks, or other relevant evidence.
2. (b)If the debtor's arguments appear to have merit, the NLRB may waive the interest period pursuant to 29 CFR 100.617(c) pending a final determination of the existence or the amount of the debt.
3. (c)The NLRB may investigate the facts concerning the dispute and, if it considers it necessary, arrange for a conference at which the debtor may present evidence and any arguments in support of the debtor's position.