Mass. Gen. Laws ch. 55C § 9
Within two weeks following any primary or state election for statewide elective office any candidate who has received public financing under this chapter shall file a statement with the director showing the balance remaining in the candidate's depository account as of the primary or state election less any reserve necessary to cover debts incurred to defray campaign expenditures incurred during such primary or state election. Except as provided in this section, any candidate having a surplus balance following any such primary or state election shall thereupon pay to the state treasurer for deposit to the State Election Campaign Fund an amount determined by multiplying such surplus balance by a fraction the numerator of which is the total amount of public financing received on account of such primary or state election campaign and the denominator of which is the sum of such public financing received and all contributions received by such candidate. No candidate having a surplus balance following the primary election shall be required to make any payment on account of such surplus if such candidate is certified by the state secretary under section 2 as qualifying for the ballot and having opposition in the state election and is certified by the director of campaign and political finance as eligible for public financing for the state election within 3 weeks following such primary election. In determining and certifying the amount to which any such candidate is entitled under section 7, the director shall reduce the amount that would otherwise be determined under that section by an amount equal to the amount that such candidate would be required to pay to the state treasurer under this section but for this sentence. If the director determines that any portion of the payments made to an eligible candidate under this chapter was in excess of the aggregate amount of the payments to which the candidate was entitled he shall so notify the candidate and he shall thereupon pay to the state treasurer an amount equal to the excess amount.
If the director determines that any portion of the payments made to a candidate under this chapter for use in his primary election campaign or his state election campaign was used for any purpose other than to defray campaign expenditures in that campaign, or to repay loans the proceeds of which were used to defray campaign expenditures in that campaign, the director shall so notify the candidate and he shall thereupon pay an amount equal to the full amount so used to the state treasurer without regard to the source of the funds so used.
Any candidate who fails to pay an amount determined by the director to be due within 10 days of notice thereof shall be subject to a penalty of $50 per day for every day that such amount remains unpaid.
Mass. Gen. Laws ch. 55C, § 9