U.S. consumers wanting to use the cash-for-clunkers program to buy a car, but can’t find the models they want due to stock shortages, will be able to reserve models, according to a law amendment by the Obama administration.

The move comes after Michigan lawmakers raised concerns that automakers were running short of popular models that would be hard to replenish before the program ends. The top-selling SUVs and trucks under the program as the Ford Escape, the Jeep Patriot, the Ford F-150 and the Chevy Silverado 1500. New data suggests half of the $2.95 billion set aside for the program will be claimed within days, and that all of the money may be exhausted by the beginning of September.

Under the original program rules, owners of clunkers who qualified for $3,500 or $4,500 vouchers had to take delivery of a new vehicle from dealers’ stock, and only after the sale was complete could dealers file for reimbursements.

The U.S. Department of Transportation now says that dealers can reserve a vehicle from a manufacturer by using a vehicle identification number, and submit that number with the rest of the paperwork to the government.

As of today, 338,659 deals have been entered into the system, with $1.418 billion in vouchers claimed. At the rate of the past few days and based on the average claim, the program would run out of money in about 15 days, although some analysts have suggested demand could slow down by then.