If you have previously entered a retainer for your client into TurboLaw Time and Billing (see: How to Enter a Retainer), then when you create a new bill for your client all you need to do is click "Save & Close" and TurboLaw Time and Billing will prompt you to pay the bill from the client's retainer account. Simply click "Yes" when prompted and TurboLaw Time and Billing will automatically deduct the correct amount from the client's retainer funds.
If the client's retainer has sufficient funds to cover the amount of the bill, then the bill will be paid in full and automatically closed.
If the client's retainer does not have sufficient funds to cover the amount of the bill, then TurboLaw Time and Billing will use whatever amount is available to partially pay the bill. The bill will remain open in this case, and if you later receive and enter additional funds from your client, you can apply them to the bill to finish paying it off, at which point it will be automatically closed.
Note: if you have entered the retainer under a matter for the client, but have not created the bill under the same matter, you will not be able to use the retainer to pay the bill - funds entered in a matter can only be used to pay bills which are also entered in the same matter. (Bills which have no matter selected are also considered in a different matter for this purpose.)