The SNP minister told MSPs a "minor technical change" is needed from UK ministers to avoid the Marriage Allowance being denied to many couples north of the border when his overhaul of the Scots tax system is implemented this year.
Workers earning more than £24,000 will pay a new 21p "intermediate" rate, under the SNP's plans, meaning they lose out on the allowance which applies to basic rate payers. Couples on the new 19 pence starter rate will also lose out.
Mr Mackay told Holyrood's finance committee today married Scots should not lose out.
"My view is that married Scots should continue to have that entitlement," he said
"It's then back to the Westminster Government to make that change or not. They would not lose out by making the change. For them it would be continuity."
Mr Mackay insisted it was "not a reason" to stop Scotland from making different decisions on tax policy.
"At maximum of £260 per couple in relief for those affected and could be resolved in advance of the new financial year with a minor technical change if the UK Government wishes to do it."
The marriage allowance applies to basic rate taxpayers if their spouse is out of work or in a low-income job.
The tax changes in Scotland mean that the current 20 pence basic rate, applying to those making about £11,501 to £43,000 will instead be covered by three rates. There will be a new "starter rate" for those making, £11,850 to £13,850, while the new 20 pence basic rate, which will retain the marriage allowance right, applies from £13,850 to £24,000. There will also be a new 21 pence intermediate rate for those earning between £24,000 to £44,273.
Aidan Grisewood, Deputy Director in the Fiscal Responsibility Division at the Scottish Government told MSPs today that talks are underway with Westminster in an effort to resolve the impasse.
He said: "The question is, in terms of marriage allowance, do you stick to the letter around the basic rate, which means that those people on the intermediate rate lose that entitlement, or do you take a pragmatic approach that avoids that eventuality.
"So we're working closely with the UK Government. We understand that essentially there's a minor legislative change that could be put in place that could be corrected for this - that again is in the UK Government's gift to take that forward."