A letter contract may be used only after the head of the contracting activity or a designee determines in writing that no other contract is suitable. Letter contracts shall not—

(a) Commit the Government to a definitive contract in excess of the funds available at the time the letter contract is executed;

(b) Be entered into without competition when competition is required by part 6; or

(c) Be amended to satisfy a new requirement unless that requirement is inseparable from the existing letter contract. Any such amendment is subject to the same requirements and limitations as a new letter contract.