In a major concession, Treasury Secretary Donald T. Regan said yesterday he is willing to consider allowing banks to hold money withheld from savings account interest for longer than 30 days.

Regan, attempting to defuse the banking industry lobbying campaign for repeal of a law requiring them to withhold 10 percent of savers' interest income, said in a letter to Sen. Robert J. Dole (R-Kan.), chairman of the Finance Committee:

"If . . . it can be reliably demonstrated that the true incremental costs that most banks must incur exceeds the value to the banks of the extended 30-day float . . . then I would support allowing the extended float for a longer time." The longer the banks hold the withheld money during the "float," the more they can make use of and profit from the money.

At the same time, however, Regan charged that the banking industry claim of $3 billion in new costs resulting from withholding on interest and dividends is based on "exaggerated estimates of the costs." The Treasury has estimated the cost at about $650 million.

In addition, Regan said that an attempt to replace the withholding requirement with beefed up enforcement by the Internal Revenue Service would cost more than $1 billion just to raise $3 billion in additional taxes collected.

The enforcement, in turn, would require the IRS to perform 2 million more audits a year, Regan claimed.