Is it permissible to adopt an arbitrary exchange rate (70 dinars per 100 dollars) without the explicit consent of the other party, and would the price difference be lawful for the questioner?

It is permissible for someone who owes a debt to repay it in a currency other than the currency of the debt, if they agree with the creditor on that at the time of repayment. The exchange rate will be according to the rate on the day of repayment, and it is a condition that the exchange takes place immediately in the same sitting. This is based on what was narrated from Ibn Umar (may Allah be pleased with both of them) that he used to sell camels for dinars and take dirhams, and vice versa. He then asked the Prophet (peace and blessings be upon him) who said: "There is no harm in it if it is at the rate of its day and you do not separate while there is still something between you." It is not permissible to agree beforehand to repay in another currency, because that is deferred currency exchange (sarf), which is usury (riba). If the debtor pays more than the debt in another currency, the creditor should take their right according to the rate of that day, and keep the excess as a trust for the debtor in the same currency that was paid.