Is it permissible for me to pay an electrical appliance vendor’s bill of 5000 with a 500 discount if I pay now, in exchange for me taking 250 and him taking 250, then he repays the full amount to me after a month?

We understand from the question that you want to pay for a device for someone in order to get a discount, and then this person repays you the amount you paid with an increase. This is not permissible because it is a loan with interest, which is usury (riba). The Shari'ah-compliant solution is for you to buy the device for yourself first, and then sell it to him at the agreed-upon profit, as indicated by the hadith of Bilal in Al-Bukhari.