Selling RSUs what tax will I pay?

I have some Shares that have vested over a year ago and less than a year ago. The tax was taken from my remaining RSUs when they were vested. Company is US based. We have fair market value which is lower than the offer price the company will buy the RSUs. My shares were vested at a price lower than the offer price but higher than the Fair market value for most of the vested shares, I had some shares vested at a lower price than the Fair market value. As The company will now buy some RSUs, I want to sell the amount of RSUs I have been allotted. I want to know if I will pay any Tax on selling the RSUs. Speaking to different people I have been told different things: 1. pay tax on the Sale price minus the Fair market value of the number of shares 2. pay tax on the Sale price minus the Sale Price I want to know if I will be liable to pay Income tax again on the sale From this article it seems as I will pay no tax on the sale of the RSUs https://frazerjames.co.uk/rsus-a-tech-employees-guide-2/#:~:text=The%20UK%20tax%20treatment%20for,transfer%20the%20liability%20to%20you. Please can someone confirm to me if I am liable to pay any tax on the sale of the RSUs. kind regards 

Name removed admin .

Hi
As the shares have already vested, when you sell them, any increase in value will be classed as a capital gain and taxed based on the gain made and your level of income.