Hi- this is a largely UK site and it sounds as if you may be outside the UK.
I'm not sure what you mean. You cannot cash a cheque for $8500 unless you have sufficient cleared funds in your account. (What constitutes 'sufficient' depends on what overdraft or minimum balance arrangements you have).
..or do you mean you have received a cheque for $8500 and you want to know if you can convert it to cash rather than pay it in.
I depends on your bank but I think they will want you to pay teh cheque in and wait until it's cleared- although you seem to have enough to withdraw $8500 at the same time as you pay in your $8500 cheque.
If you are saying that you have a cheque in your hand for £8500, it will depend on your bank whether they will give you the money rather than making you pay it in first - I'd say not, they will want to know that the cheque will clear first.
No, I doubt very much that they would exchange your cheque for cash until it has cleared,
You could withdraw some of your existing $10000 balance )provided the funds are cleared funds) but your bank may have a limit on how much cash it can pay out to someone in one day.
You really need to ask your bank.