Customer over paid invoice, would like his credit to go to another customers invoice

Hi ShellyMac1, 
 
Thanks for your post. Please allow me to extend to you a very warm welcome to the Community Forum. We are thankful for your effort in notifying us about your inquiry. We apologize that you are facing issues applying the credit to an invoice and we appreciate your patience and understanding on this matter. 
 
Regarding your concern, you may check and follow the steps on our Help Articles to apply a credit to an invoice: 
 

• Customer returns, credits, and refunds.
• Settling customer credits.

Please do not hesitate to post again if you need help in the future. 
 
If my response has answered your enquiry, please click "Accept as Solution" to assist other users find this information. 
 
Cheers,  
Leneth