What is the ruling of Islamic law on settling the inheritance of daughters at a price lower than the current market value, with their consent and forgiveness, and are they obligated to take their full share even if they forgive?

What your brother advised you is correct because Allah the Almighty Himself undertook the division of inheritances in His Book, and did not entrust this matter to anyone. He concluded the verses (on inheritance) warning against violating them, and clarifying the punishment for whoever transgresses His limits. Allah has clarified the right of daughters in His saying: "To the male, a portion equal to that of two females." Therefore, you must divide the inheritance according to what Allah has commanded, and no heed should be paid to any custom that contradicts this.

If the sisters willingly consent and relinquish their right, or part of it, then there is no sin upon you in taking it. However, if their relinquishment was due to shyness rather than willing consent, then it is not permissible for you to take it; for what is taken out of shyness is like what is taken by force. Taking by force is of two types: forceful seizure and taking by shyness, and both are forbidden.

We advise you to give your sisters their full and undiminished share of the inheritance, for the criterion is that the male receives a portion equal to that of two females, by any calculation.