Is it permissible to increase the amount of a debt in an installment sale, then waive this increase if the installments are paid on time, and demand the full amount if there is a delay?

It appears that the price of the commodity at the time of the contract was not specified; rather, it fluctuated between two prices, one for one condition and the other for another condition. This invalidates the contract, because one of the conditions for a valid sales contract is the determination of the price at the time of its conclusion. Ambiguity in this regard constitutes gharar (uncertainty) that invalidates the contract. This falls under the category of "two sales in one sale," which is forbidden. For example, if one says: "I sell you this garment for ten [cash], or for twenty [on credit] until a month," this is invalid according to most scholars due to the ambiguity of the price.