Should I return simple tools (not exceeding four dollars in value) that I obtained from my previous job, or donate their value with the intention of redressing grievances?

You must return the tools to their owner if he did not gift them to you, based on the generality of Allah’s saying: "Indeed, Allah commands you to render trusts to whom they are due," and the Prophet's (PBUH) saying: "On the hand is what it took until it renders it." It is not sufficient to donate their value in charity unless you are unable to deliver them to him.