Must my pay and conditions match those of Bulgarian workers on a single permit?

A core protection in the single permit scheme is equal treatment on pay and working conditions. The terms offered to you must not be worse than those that apply to Bulgarian nationals in the same category of work.

This is designed to prevent third-country workers from being used to undercut local wages and to ensure fair treatment. Before you accept an offer, compare the salary and conditions against what is normal for the role in Bulgaria, and make sure the contract reflects that equal-treatment standard - it is both your right and a condition of the permit.