Am I to be held sinful, or liable for expiation, or for restitution of money if a customer's update was not processed due to a glitch on the website, knowing that I did not intend for that to happen?

Reducing the price of a commodity for the buyer is permissible, and it is one of the commercial incentives to encourage people to buy, so long as it does not lead to something forbidden, such as deceiving buyers or harming other sellers.

If the merchant and the buyer agree upon a discount, then the seller is not entitled to anything more than the named price after the discount. If the discount was not applied due to an error, then the seller must pay the amount of the discount to the buyer. If the buyer is unaware, then the seller should give it away as charity on their behalf.