Nothing in this chapter shall be construed to constitute authorization hereunder for—
((a)) the use of the cost-plus-a-percentage-of-cost system of contracting;
((b)) any contract in violation of existing law relating to limitation of profits;
((c)) the negotiation of purchases of or contracts for property or services required by law to be procured by formal advertising and competitive bidding;
((d)) the waiver of any bid, payment, performance, or other bond required by law;
((e)) the amendment of a contract negotiated under section 2304(a)(15) of title 10 or under section 252(c)(13) of title 41, to increase the contract price to an amount higher than the lowest rejected bid of any responsible bidder; or
((f)) the formalization of an informal commitment, unless it is found that at the time the commitment was made it was impracticable to use normal procurement procedures.