Advice

So I have a best friend whom I might move in with as their current roomie is moving out real soon. Anyway they offered to charge me $400 in exchange for helping cleanup etc. anyway they would give me back $100 out of the $400 and told me it would have to be put into my investment account(money is already automatically put in there thru my bank account with every purchase etc) anyway I said why not just charge me $300 instead of $400 and if my cleaning isn’t up to their standards they can charge me another $100. It should be my decision on whether or not I want to put money in that account, not theirs right? Am I wrong? Anyways I told them that and they said if the decision is left up to me, they aren’t sure they see it happening(me moving in)