What is the ruling on the extra amount (22,0 EGP) that was paid for the purchase of marriage furniture without my knowledge or prior agreement? Is it considered a debt upon me? If so, how can the cost of the airline tickets (1050 SAR) that I later paid for my mother-in-law be settled, taking into account the change in currency exchange rates?

If you agreed with your mother-in-law to purchase furniture for a specific amount, and she bought it for more than that without your permission, you are not obligated to accept the increase unless you ratify it. The majority of scholars hold that the action of an unauthorized agent (fuduli) is valid if the owner ratifies it. Since you accepted the furniture, you now owe your mother-in-law 22,0 pounds.

If you agreed with your mother-in-law that her travel tickets would be her responsibility, then she owes you 1050 Riyals. Regarding a monetary debt, the principle is that it should be repaid with its equivalent. Since the value of the Riyal has not decreased, your mother-in-law's debt to you in Riyals is not affected. As for your debt to your mother-in-law in Egyptian Pounds, you must consider the value of the 22,0 pounds at the time of the debt and what amount of gold it was equivalent to, so you pay her now what buys the same amount of gold.

You both may agree to an offset at the time of settlement, so that what she owes is deducted, and the remainder is paid to her.