Q: I have a client for whom I do short jobs; to avoid frequent invoicing and small bank transfers, he simply pays me a sum in advance. As I do jobs, I credit the advance fee till it is gone and then he sends me more money. 

Can I account for such an arrangement in TO3000? Thanks for your ideas!

A: To implement the scheme in question you can add payments from your client at the Client - Payments tab,
then gradually add the jobs you do for the client to the jobs list at Project - Client Jobs.

After you create the job you can create an invoice for that job at Client - Invoices - New. 
After the invoice is created, proceed to Client - Payments, open the payment you've added upon receiving the advance from your client, switch to "Linked with invoices", click "new link" and link the payment to your invoice.
Click OK to confirm changes.