(a) The receipts from the sale, use, or rental of and the storage, use, or consumption of taxable items in this state are exempt from the tax imposed under this chapter by a taxing entity created under Chapter 453, Transportation Code, if the items are used:
(1) for the performance of a written contract entered into before the date the tax takes effect in the taxing entity, if the contract is not subject to change or modification by reason of the tax; or
(2) pursuant to an obligation of a bid or bids submitted before the date the tax takes effect in the taxing entity, if the bid or bids may not be withdrawn, modified, or changed by reason of the tax.
(b) The exemptions provided by this section have no effect after three years from the date the tax takes effect in the taxing entity.