this is the original code :

/** @odoo-module **/

import { _t } from "@web/core/l10n/translation";
import { ProductScreen } from "@point_of_sale/app/screens/product_screen/product_screen";
import { useService } from "@web/core/utils/hooks";
import { NumberPopup } from "@point_of_sale/app/utils/input_popups/number_popup";
import { ErrorPopup } from "@point_of_sale/app/errors/popups/error_popup";
import { Component } from "@odoo/owl";
import { usePos } from "@point_of_sale/app/store/pos_hook";

export class DiscountButton extends Component {
    static template = "pos_discount.DiscountButton";

    setup() {
        this.pos = usePos();
        this.popup = useService("popup");
    }
    async click() {
        var self = this;
        const { confirmed, payload } = await this.popup.add(NumberPopup, {
            title: _t("Discount Percentage"),
            startingValue: this.pos.config.discount_pc,
            isInputSelected: true,
        });
        if (confirmed) {
            const val = Math.max(0, Math.min(100, parseFloat(payload)));
            await self.apply_discount(val);
        }
    }

    async apply_discount(pc) {
        // here i want to add som validation
    }
}

ProductScreen.addControlButton({
    component: DiscountButton,
    condition: function () {
        const { module_pos_discount, discount_product_id } = this.pos.config;
        return module_pos_discount && discount_product_id;
    },
});

and this is my code 

/** @odoo-module **/

const _t = require("@web/core/l10n/translation")._t;
const ProductScreen = require("@point_of_sale/app/screens/product_screen/product_screen").ProductScreen;
const NumberPopup = require("@point_of_sale/app/utils/input_popups/number_popup").NumberPopup;
const ErrorPopup = require("@point_of_sale/app/errors/popups/error_popup").ErrorPopup;
const Component = require("@odoo/owl").Component;
const usePos = require("@point_of_sale/app/store/pos_hook").usePos;
const DiscountButton = require("@pos_discount/overrides/components/discount_button/discount_button").DiscountButton;

export class CustomDiscountButton extends DiscountButton {
    async apply_discount(pc) {

        console.log('some validation');

        await super.apply_discount(pc);

        
    }

    async click() {
        var self = this;
        const { confirmed, payload } = await this.popup.add(NumberPopup, {
            title: _t("Discount Percentage"),
            startingValue: this.pos.config.discount_pc,
            isInputSelected: true,
        });
        if (confirmed) {
            const val = Math.max(0, Math.min(100, parseFloat(payload)));
            await self.apply_discount(val);
        }
    }
}

ProductScreen.addControlButton({
    component: CustomDiscountButton,
    condition: function () {
        const { module_pos_discount, discount_product_id } = this.pos.config;
        return module_pos_discount && discount_product_id;
    },
});