Sheffield firm Ashton Morton Slack has been forced to repay £1.45m of the money it has made from representing sick miners.
The repayment is the result of a High Court ruling earlier this year that firms had been overpaid for processing claims.
However, the firm will offset the money against future miners' scheme-related earnings rather than pay it in a lump sum.
Partnership secretary Ian Mitchie said the firm had written a cheque for £1.45m, but then decided it was more cost-effective to earn nothing from future cases until it had repaid the debt.
"With interest rates as they are, we were looking at effectively paying £150,000 in interest as well," added Mitchie.
Last Friday (31 August) was the deadline for firms involved in the miners' litigation to decide whether to repay money owed in a lump sum or via the Department for Business, Enterprise & Regulatory Reform's offsetting scheme.