There is no need to do this. If you use the
Billable Expenses module, the expense reimbursement never hits an income account. The expenses show as a distinct asset until invoiced, then move to
Accounts receivable, and are credited back to zero when receipt of money in payment of the invoice debits a cash account. This is different from billable time, which immediately shows in an income account when entered, despite not yet being invoiced.
All this is invisible to the client, who might see a sales invoice with line items originating in Billable Time, Billable Expenses, and other sources. A single receipt clears them all properly and keeps the taxable income separate from pass-through expenses.
For those in the USA, the challenge is make sure the customer reports only the taxable portion of the payment in Block 7 of Form 1099 MISC. The government instructions are clear about this, but many customers don’t read them and lump all payments into the same category. In that case, you can ask for a corrected form or enter a special expense category on your tax return in offset.