Finding a definition for the "por ajuste alzado riguroso" was the fun part of this.

I think the Google Books link, the first web reference, explains it well. In this case, what the contract is implying is that the price for the labor is set and cannot be modified, even if the subcontractor's costs run over budget, in which case we'd have a "cost-plus contact" a.k.a. a "cost reimbursement contract," which allows for repayment of costs over the original budget.

When translating "contrato" a lot of times it is rendered as "agreement," but I'm finding more sources web sources with "contract." For example, this link to U.S. Veteran's Affairs makes mention in item 852.236-82:

http://www.va.gov/oal/library/vaar/vaar852.asp

I hope this helps!