Are you referring to the 3-2 Receivable Invoices screen?

Are you intending to use lump sum contracts that have sales tax included?

I"m referring to the 3-5 screen when you look at the contract summary button. We process invoices against a job that have sales tax which is included in the original /new contract amount. However, sales tax is not included in the Invoiced to Date field so it appears that the contract has not been paid in full; but it has. When you look at the "invoices" button, invoices will equal the original/new contract amount because they reflect the amount with sales tax. With this discrepancy, it also shows up on other reports that make the job look like it has not been billed completely.

We exclude tax in the Contract Amount field. In the notes, we put "contract amt inc tax $xxx" so that anyone looking in the 3-5 screen can see both amounts. You don't want WIP calculating on the taxed amount is it beneficial to put the pretax amount in the Contract Amt field.

The deciding factor on whether to have it added to invoices separately (which accrues it to a liability account on your balance sheet) is if you collect the sales tax and then submit to the state.
Typically fixed price construction does not involve collecting and submitting that tax separately to the state.
So if you pay taxes on the materials you purchase then you typically do not charge your customers sales tax because its already been paid. If this is the case, then you simply need to bill out 100 of your contract including the sales tax (but don't list it separately on the invoices).

This info comes from the CA BOE website:

Fixed-price contracts and district taxes.
To qualify as an exempt contract from district taxes, a “fixed-price contract” must meet the following conditions:

It must have been entered into before the effective date of the district tax.
Exception: A contract signed after the effective date qualifies if (1) it is based on a binding bid submitted before the effective date, and (2) it is signed before the bid can legally be changed.
Contracts for which an irrevocable (binding) bid was submitted prior to the effective date will qualify even when signed on or after that date provided they are signed during a period for which the bid is still irrevocable.
It must be for a fixed amount. A contract may not have any clauses that allow for increases or decreases in the contract price because of a change in the tax rates or the cost of the property to be furnished. This provision is not invalidated if the contract allows change orders. Change orders are considered separate contracts.
All parties must be bound by the terms of the contract.
No party can have the unconditional right to terminate the contract, whether or not that right is exercised.
The sales tax amount or rate must be specifically stated in the contract or the contract must state it is tax-included.
Leases and construction contracts
Leases and fixed-price construction contracts may qualify for this district tax exemption.
A time-and-material construction contract does not qualify.
Please refer to publication 9, Construction and Building Contractors, and publication 46, Leasing Tangible Personal Property.
The exemption allowed for fixed-price contracts applies not only to sales of materials and fixtures under fixed-price construction contracts and to contracts for leases of tangible personal property but also to standard retail sales contracts. These publications can also be mailed to you by contacting our Customer Service Center at 1-800- 400-7115.

When you use tax feature sage only books the pretax amount to sales but the entire amount to AR and to invoices to date. I don't understand why you would book the net as the original contract amount when it really isn't.
Are you confusing that with budget?