Is it permissible for the borrower to pay a portion of the remaining amount owed to the lender before the installment payment period ends?

If the relationship between the questioner and his brother is that of a borrower and a lender, such that the brother lent him 34,000 and the questioner will return 43,000, then this is a loan that drew benefit for the lender, which is usury (riba) by scholarly consensus. Both of them must repent from this, and the questioner should return only the original amount without any increase, in accordance with the Almighty's saying: "And if you repent, you may have your principal – [thus] you do not wrong, nor are you wronged."

However, if the brother relinquished the shares at their original price, and the questioner sold them as he wished, then this is an ordinary transaction called "tawliyah" (resale at cost price), which is permissible in Islamic law, and the questioner should return the original price to his brother.