The International Advisory and Monitoring Board said the work by Halliburton had been either overpriced or insufficiently documented.
Its recommendation came after it conducted an audit on contracting work by Halliburton's KBR unit in 2003-04.
In response, the energy firm denied overcharging and insisted it had co-operated with the audit.
The IAMB has been set up to monitor the spending of Iraq's oil revenues, following the US-led invasion in 2003.
It can make recommendations but not decisions on whether reimbursements are made.
(More ... Audit Says US Should Repay Iraq | Americas | BBC News)