Depends on the contract - if the contract states "a washing machine" is provided then 7kg is not only more than fair, it's generous of you to pay extra. If the contract states "an 8kg washing machine" is included, then you would have to replace it.

FWIW, I once rented a house with a dishwasher and was told "when it goes, it goes and won't be replaced". That was fine. Aside from essentials (such as a boiler) then you should check if you have to replace at all or whether the renter should buy one and take it when they go. (From someone who kitted out a rental place with washing machine and freezer - which went with her as she left - as they weren't provided as they aren't essentials).