The state’s top lawyer is being asked whether an agency’s financial commitment to finish the disputed Mark Clark Expressway in Charleston was legal — and whether other state agencies could use the same methods to spend money.

In a letter this week to Attorney General Alan Wilson, Rep. Robert Brown, D-Charleston, asked whether the state Infrastructure Bank could legally commit future bonding capacity for the more than $500 million project.

In August, the state Infrastructure Bank agreed to provide up to $150 million in future bonding capacity to complete the extension of the Mark Clark Expressway. The Infrastructure Bank already had committed $420 million and was out of current borrowing capacity.

Freeway boosters said the decision means a badly needed road will be built, but the move drew criticism from others who said the freeway extension is too expensive, unneeded and environmentally damaging.

Brown’s request for an attorney general’s opinion is an attempt to settle legal questions that arose from the bank board’s decision, he said Thursday. Brown wants to know whether other South Carolina agencies can make the same future bonding commitments.

Wilson spokesman Mark Powell said the attorney general’s office has received the request, but such opinions usually take time.

Critics have likened the Infrastructure Bank’s action to that of someone whose credit cards are maxed out, who then promises to pay on credit in the future after the bills are paid down.

“We are spending money we don’t have and now we are borrowing our grandchildren’s money,’’ Brown told The State. Brown’s request also contains other legal questions tied to the controversial Charleston County project.

The state Infrastructure Bank is a tiny, independent road-funding agency that has committed billions of dollars for major highway projects in South Carolina. It is not a part of the state Department of Transportation.

Several bills have been filed in the Legislature this year to dissolve the Infrastructure Bank board and assign its duties to the DOT, including one filed Thursday in the S.C. House by Rep. Ralph Norman, R-York.

Board critics say it has sent most of the money it controls to Charleston, Horry and Greenville counties. Counties such as Richland have gotten little or nothing from the bank. Boosters of the bank say it was only responding to requests and the need for major highway projects.