Apply Credit from My Company to a Customer

I sponsored an event for AB company; all the expenses I incurred were marked as billable; I have created an invoice successfully and it looks as expected.

For this event, my company received a donation to help cover some of these expenses; I created a Sales Receipt to accept the payment and deposited it to my checking account.

How do I pass this donation/credit on to AB company and deduct it from the invoice? Do I use a negative amount on the invoice - what account do I use? Do I issue them a Credit Memo, again what account. Or is there a better way I haven't mentioned?

Thanks!