48 CFR §43.102

1. (a)Only contracting officers acting within the scope of their authority are empowered to execute contract modifications on behalf of the Government. Other Government personnel shall not—
   1. (1)Execute contract modifications;
   2. (2)Act in such a manner as to cause the contractor to believe that they have authority to bind the Government; or
   3. (3)Direct or encourage the contractor to perform work that should be the subject of a contract modification.
2. (b)Contract modifications, including changes that could be issued unilaterally, shall be priced before their execution if this can be done without adversely affecting the interest of the Government. If a significant cost increase could result from a contract modification and time does not permit negotiation of a price, at least a ceiling price shall be negotiated unless impractical.