14 CFR §151.31

1. (a)An offer by the Administrator, and acceptance by the sponsor, as set forth in § 151.29, constitute a grant agreement between the sponsor and the United States. Except as provided in § 151.41(c)(3), the United States does not pay, and is not obligated to pay, any part of the project costs that have been or may be incurred, before the grant agreement is executed.
2. (b)The Administrator and the sponsor may agree to a change in a grant agreement if—
   1. (1)The change does not increase the maximum obligation of the United States under the grant agreement by more than 10 percent;
   2. (2)The change provides only for airport development that meets the requirements of subparts B and C; and
   3. (3)The change does not prejudice the interests of the United States.
3. (c)When a change is agreed to, the Administrator issues a supplemental agreement incorporating the change. The sponsor must accept the supplemental agreement in the manner provided in § 151.29(c).