(a) The governing body of a municipality that issues bonds under this subchapter shall establish rates for sewer service in amounts sufficient to:
(1) pay:
(A) maintenance and operation expenses;
(B) the bonds as they are scheduled to mature; and
(C) interest on the bonds as it accrues; and
(2) establish and maintain any fund provided in the ordinance authorizing the bonds.
(b) Notwithstanding Subsection (a), the municipality may not, during the term of a contract for sewer service, increase the amount of the consideration for that service specified in the contract except:
(1) as the contract provides; or
(2) as the parties to the contract agree.