I've got the steps on how we can apply the credit memo, 

WCSW.

Let me guide you with these steps:

1. From the Customers menu, select Credit Memos/Refunds.
2. Select a CUSTOMER:JOB.
3. Select Donations under the ITEM field.
4. Enter the amount of $2500 on the AMOUNT section.
5. Click Save & Close.

Then, here's how to link the credit memo and the invoice:

1. Click Receive Payments on the Customers menu.
2. Select the same customer's name on the RECEIVED FROM field.
3. Select the invoice and the credit memo.
4. Click Save & Close.

I'm just around if ever you need help. Keep safe!