Can coupons be used when a seller makes you an offer?

Ebay seller has made me an offer on an item I have in my basket for ~ £15 less than the listing price.

However, I have a discount code applied in my basket that reduces the price to considerably lower than their new offer.

Does anyone know if the discount code would still be applicable to the item if I accept their offer?

I don't want to accept it and find it works out more expensive for me.

TIA