I had a similar situation at my previous firm:

Caught sight of my colleague's e-payslip once when she asked me to help fixed her hanged computer. Realised that she was getting 20% more than me.

1 week later. I went to negotiate with boss. Said I received an offer 1.3x my current pay. (of course cannot mention that I caught sight of colleague's payslip but did a round-about nego tactic)

Boss agreed to match 1.3x but 2 weeks later said HR approved only 1.2x.