日本法の下では、同趣旨の表現を単語ではなく、下記のようにentrusted/entrusting operationとしているようです。下記翻訳は、法律の公式な翻訳です。
(It seems that under the Japanese laws, it is expressed not as a single word but instead as a phrase. The translation below is an official translation cited from a law.)

下記航空法施行規則 第二百二十二条 第二項
一　管理（業務）の委託及び受託契約書の写し
(i) Copy of the contract of entrusted/entrusting operational control.

It's difficult to judge because of lack of context, but what I think you mean is a middle man, who is contracted by the end customer and then entrusts work to a subcontractor.

In this case, I think "primary contractor" may be the term you are looking for.
FYO:

http://www.ehow.com/info_8469298_prime-contractors-vs-subcon...