I'm curious how to I assign a payment from a client after I sent them an invoice.

It's nice to have you in the Community, @daniel-hoythouse. I'd be happy to help get this taken care of today.

You can use the receive payment feature to record the invoice payment received once your customer pays. Here's how:

1. In the left menu, click the + New button. 
2. Choose Receive Payment.
3. Pick the name of the customer in the drop-down menu and enter the payment date. 
4. Fill out the Payment method, Reference no., and where you'll like to deposit the money in the Deposit to field. 
5. From the Outstanding Transactions section, select the transactions that you’d like for QuickBooks Online to calculate. 
6. Once done, select Save and close. 

However, if you use QuickBooks Payments, once your customer pays, we will handle everything and categorize it to the right account. 

For more information about receiving and categorizing invoice payments, see this article: Record invoice payments in QuickBooks Online. 

Adding you also some resource which you may find helpful:

• Take and process payments in QuickBooks Online
• Process a credit card payment in QuickBooks Online
• How to record an invoice payment (using undeposited funds)

You can always get back to me whenever you have other questions. I'm always here to help. 

Hi @MirriamM 

I actually sent them an invoice outside of quickbooks, so I don't know if I can actually assign their payment to match an invoice number. What I'd like to accomplish is when I see a credit of say $10k, I can assign that amount to a project/client so it can help track my profitability on the current project. Thank you so much for your help!

Hello there, daniel-hoythouse.

You might want to record the invoice you sent outside QuickBooks into the system. Once you receive the customer's actual payment, you can also pay that invoice using the Receive Payment option that was shared by my colleague. 

If your bank is connected in QuickBooks Online, you can match it to the payment you've recorded earlier. For more details, feel free to check these references that were also provided by MirriamM: 

• How to record a bank deposit (using undeposited funds)
• Categorize and match online bank transactions in QuickBooks Online

Let me know if you have other questions. Take care!