Del. Code tit. 19 § 3357
Assessments and reimbursement payments in lieu of assessments which remain unpaid on the date they are due and payable, as prescribed by the Department, shall bear interest, at a rate determined by regulation by the Secretary of Labor, from and after such due date until payment plus accrued interest is received by the Department. Interest collected pursuant to this section after September 30, 1967, shall be paid into the Special Administration Fund.
The Department may waive the payment of interest for a period of not more than 6 months in cases where it appears to the satisfaction of the Department that the delayed payment was caused by a reasonable doubt as to liability and the employer was not negligent in applying for a determination of the question of liability.
No interest shall be charged in cases where the Department has ascertained that an amount equal to the amount of the delayed payment had previously been paid by the employer into the unemployment trust fund of another state.
19 Del. C. § 3357