trigger DiscountOpportunity on Opportunity (before insert,before update) {
        Map<Decimal, Decimal> discountTiers = new Map<Decimal, Decimal> 
        {
        100000 => 0.10, 
        50000 => 0.05 
    };

    Decimal maxDiscount = 20000;

    for (Opportunity opportunity : Trigger.New)
    {
        Decimal discountAmount = 0;

        for (Decimal tierAmount : discountTiers.keySet())
        {
            if (opportunity.Amount >= tierAmount)
            {
                discountAmount = opportunity.Amount * discountTiers.get(tierAmount);
                break;
            }
        }

        if (discountAmount > maxDiscount) {
            opportunity.Discount_Amount__c.addError('Maximum discount exceeded!');
        } else {
            
            opportunity.Discount_Amount__c = discountAmount;
        }
    }

}