Is the consent of surety required for a modification that reduces the construction contract by more than 50,000? No additional bond is required and the modification does not change the contract scope but changes the contract price ( downward) by more than 25 percent or $50,000.
The requirement of when to obtain a Consent of Surety can be found at FAR 28.106-5 -- Consent of Surety. If you read (a)(2)(ii) below, this directly addresses your question.
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(a) When any contract is modified, the contracting officer shall obtain the consent of surety if--
(1) An additional bond is obtained from other than the original surety;
(2) No additional bond is required and --
(i) The modification is for new work beyond the scope of the original contract; or
(ii) The modification does not change the contract scope but changes the contract price (upward or downward) by more than 25 percent or $50,000; or
(3) Consent of surety is required for a novation agreement (see Subpart 42.12).
(b) When a contract for which performance or payment is secured by any of the types of security listed in 28.204 is modified as described in paragraph (a) of this subsection, no consent of surety is required.
Based on the scenario you presented and FAR 28.106-5(a)(2)(ii), the contracting officer is required to obtain a consent of surety for your modification reduction.