Sun opposite Mercury transit has a stimulating effect on mental activity. Your interactions with other people are brought into focus. However, your thinking may not align with your goals and your ego; conflict and arguments could result if you push too hard, as you are likely to speak without thinking and stir up opposition.

You are thinking more about yourself, and your views are likely to be narrow, which is what makes others get their backs up. The key to handling this difficult energy is to listen more than talk and remember to think twice before making statements or expressing personal opinions that might offend.

This is not the best time to ask for a raise from the boss or to try to reach a deal in business. Things will go more easily if you keep selfish thoughts to yourself. Make a conscious effort to give some ground in any arguments, at least till a more favorable time when your thinking is more balanced.

This stirres up many Accusations and Criminations against the Native, as counterfeiting some Writings, or guilty of such abusive Actions; denying falsly what may be legally proved, whereupon much Infamy fals upon the Natives discredit either by not paying Moneys, or by forgery, or counterfeiting or clipping Moneys; the Mind extreamely afflicted, and losse of Office, if it be in bawdy Courts, or Spirituall; an aversenesse to Study; oppressed with Clamours, and variety of unjust Asperions.
      How the Sun can come to the opposition of Mercury, I confesse my selfe ignorant, unlesse men live almost one hundred yeers or more. [Christian Astrology, William Lilly, 1647, p.685-686.]

This interpretation for the Sun opposite Mercury transit can also be applied to a Solar Eclipse and a New Moon opposite Mercury.