Is the consultant sinful regarding the price of land if he leans towards the weak seller and the buyer does not take his advice after consulting others?

It is considered deceit and betrayal for an advisor to give their advisee an unrighteous counsel. The questioner's bias towards the seller constitutes a dereliction of trust. It does not matter that the seller is in need or is weak. Rather, it would have sufficed for him to be fair to both the seller and the buyer by stating the appropriate price, regardless of their circumstances. Believers are commanded to be steadfast in justice, fearing no blame from anyone in the way of Allah. The fact that the buyer was not content with his advice does not absolve the questioner of this error, and he must repent and seek forgiveness.