When the bank loans out 900 dollars to me and places it in a checkable deposit, then on my balance sheet it would be +900 under assets.And now this...

Every transaction has dual effect in accounts, one on
debit side and other on credit side. This is why modern accounting system are called
"double accounting systems'. The confusion described in the question is because only one
to the two aspects of the accounting effect is being
considered.

When the loan is taken and put in the checking
account, it increases the bank balance. This is on the asset side of the balance sheet.
However at the same time there has to be be a matching entry on the liability side of
the balance sheet. This will be in the form of a loan taken from the bank. The $900
increase in assets because of increase in checking account balance will exactly match
the $900 increase in liability shown as increase in loan taken from bank. Thus there is
no net increase or decrease in balance sheet. When all accounts have been made correctly
total of assets and liability sides of the balance sheet are always equal, which cancel
each other, giving net value of 0.

In the second
transaction, when the money is withdrawn from the bank, the bank balance gets reduced,
but the money would still be there in some form. It can be in the form of cash or things
purchased, or increase in expenses incurred. So, what is done with the money withdrawn
from the bank will be represents as the other aspect of the transaction. Net effect of
both these accounting entries will still be 0.

In general,
putting money in bank and withdrawing it, has no impact on net assets, It only changes
the form in which the assets are held.