Please contact Customer Finance at 800-468-8996 (option 4, sub option 2) or email blackbaudaccounts.receivable@blackbaud.com to apply an open credit to a specific invoice. 

U.K. clients can request this from our finance department at 0207 601 7100 or BBEFinance@Blackbaud.co.uk, just mention the invoice numbers you want to allocate against each other.