The best way to invoice for the retainer is through the trust account and request replenishment.

Roland:

You will want to take a look at the Timeslips Funds feature. Typically used by law firms for their IOLTA or trust accounts, you can use it to request that a client "replenish," or in your case provide the initial, retainer. In Timeslips we can track that account separately from what you eventually bill as an account receivable by using the Funds feature.

Hope this helps.