Make sure you get a signed agreement before work is started on the project.
This is important both from getting the signed contract from the client, but also to do the same if you are using subcontractors.
Make sure the agreement allows you to terminate the subcontractor
Get written agreement to all terms and conditions.
If agreement is a long process and you have to instruct the subcontractor to start some work, make a written authorization and get it signed and set a cost limit to that work.
If your company decides to begin work before getting formal written approval to proceed you must get approval from management as high up as possible in your company. You must also be aware that the risks on the project have substantially increased by not having a written contract.