Is it permissible to calculate the exchange rate on the day of writing post-dated checks when purchasing building materials in one currency and settling their value in another currency, or must it be calculated on the day the check is cashed?

It is not permissible to buy on credit with a specific currency while agreeing to pay with another currency, because it is a deferred currency exchange (sarf mu'ajjal).

However, it is permissible for the two parties to agree on the day of payment—not on the day of contracting—to pay with another currency at its exchange rate on that day.

It is not permissible at the time of concluding the contract to agree to link deferred debts to gold, or silver, or another currency, or a basket of currencies, due to the gharar (uncertainty) and jahalah (ignorance) that result from such an arrangement.