I agree with @Abeiku’s initial point. In the situation described, you have been given a permanent discount. That discount should be reflected in the cost of inventory, because no matter what happens in the future you have paid less for the goods.
Allocating the discount to an income account by entering a negative line item will balance your books. But it will also do two undesirable things:
- Add to your current taxable income. Although an offsetting reduction will eventually occur when the goods are sold, you might keep slow-moving items in inventory for years. Meanwhile, you will have paid tax on the income you did not yet receive. And later, you will claim a deduction for an expense you did not really incur.
- Add to the current value of Inventory on hand. This overstates your current assets. As mentioned above, this overvaluation is what will eventually (hopefully) give you a larger cost of goods expense. But until that happens, you will be misrepresenting your position on your balance sheet. Auditors will not like this. Bankers will not like this. And, in some jurisdictions, inventory on hand is taxable every year as a property tax, so you will not like this.
As @Abeiku wrote, normal accounting practice does not build warranty allowances into assets. It considers potential warranty costs when setting profit margins, then records warranty costs as current expenses. So in your example, the question you should be asking is how much margin do you need to add to the €50 cost when setting your sales price. If there is a 10% chance you will have to replace the item under warranty, add €5 plus your desired profit. If there is a 50% chance, add €25 plus desired profit. Any future warranty costs can then be posted to a current expense account, reducing profit and net income in the period when they occur.
One other thing you might consider is transferring some of your current net profit from Retained earnings to a warranty reserve equity account for visibility. But, understand that this would really just be a subdivision of Retained earnings.