I think you mean you have an overdraft and that the balance of your account is minus �900 so you owe the bank money.
By the power of arithmetic, if you deposit �400 you will end up with a balance of minus �500 so you will owe the bank less money but the overdraft will have swallowed your wages.
If you have an authorised overdraft, you could withdraw more money up to your limit. If not, you may have to ask the bank to extend your overdraft to cover your living expenses.
Because you have an overdraft you'll be paying interest and charges on that overdraft which will add to the debt. Check the terms and charges on your account - speak to the bank if you're not sure.
Overdrafts can be fine if they're for a short time and you can replace the funds quickly. Otherwise they can be expensive.