A service was rendered to me on behalf of a client. How do i allocate that amount to the client on the next invoice?

Hello there. 

Let's enter a billable expense to allocate the service amount rendered to you on behalf of your client to your next invoice. I'll gladly guide you through the process below.

You'll have to record and track billable expenses so your customer can reimburse them when they receive their invoice. To do this, you'll have to turn on billable expense tracking. Here's how:

1. Go to Settings ⚙, then select Account and settings.
2. Select the Expenses tab.
3. From the Bills and expenses section, select Edit ✎.
4. Turn on the Show Items table on expense and purchase forms, Track expenses and items by customer, and Make expenses and items billable options.
5. (Optional) Set up the Markup rate, Billable expense tracking, and Sales tax charge.
6. Choose the Default bill payment terms.
7. Select Save.

Once you're done, bill your client for the expense you incur and then link it to their invoice to reimburse the cost. Please see Steps 2 and 3 in this article for the detailed steps: Enter billable expenses.

Additionally, when your customer makes a payment on the invoice, record it in QuickBooks so your accounts stay balanced. You can refer to this article for the complete guide: Record invoice payments in QuickBooks Online.

If you have other questions or concerns about recording billable expenses in QuickBooks, please don't hesitate to leave a reply below. I'm always ready to help you out.